Chemical Physics 305 (2004) 155–164
www.elsevier.com/locate/chemphys
Microwave observation of H3N–SO3 H2O using a concentric,
dual-injection nozzle source
S.W. Hunt 1, C.S. Brauer, M.B. Craddock, K.J. Higgins 2, A.M. Nienow, K.R. Leopold
*
Department of Chemistry, University of Minnesota, 207 Pleasant St., SE, Minneapolis, MN 55455, USA
Received 28 May 2003; accepted 1 June 2004
Available online 24 July 2004
Abstract
The complex H3N–SO3 H2O is observed by pulsed nozzle Fourier transform microwave spectroscopy using a newly designed
concentric, dual-injection nozzle source. This source allows two reactive species to be injected independently into a seeded supersonic expansion by introducing gases through a pair of concentric hypodermic needles situated downstream of the nozzle orifice.
Microwave spectra of the parent form of H3N–SO3 H2O, as well as nine isotopically substituted derivatives are observed, and
the N–S bond length is estimated to be 1.83(13) Å. This value is in reasonable agreement with previous theoretical calculations
and suggests that the complexation of H3N–SO3 with water produces a substantial contraction of the nitrogen–sulfur bond. The
spectrum shows no evidence of internal motion of the water subunit.
2004 Elsevier B.V. All rights reserved.
1. Introduction
Understanding the mechanism for the nucleation of
sulfate aerosol is an important problem in contemporary
atmospheric science [1]. While heterogeneous nucleation
is undoubtedly the dominant means for aerosol formation
in many cases, it seems generally accepted that homogeneous nucleation may also provide a viable mechanism under appropriate conditions. An intriguing observation
that has emerged in recent years is the apparent ability
of ammonia to accelerate the formation of sulfate-based
particles in the troposphere [2–5], and indeed NH3 is a
component in the variety of polar molecules that have
been considered as possible participants in the homogeneous nucleation process. One such polar molecule is the
*
Corresponding author. Tel.: +1-612-625-6072; fax: +1-612-6267541.
E-mail address: [email protected] (K.R. Leopold).
1
Present address: Department of Chemistry, University of California, Irvine, CA 92697-2025, USA.
2
Present address: Department of Chemistry and Chemical Biology,
Harvard University, 12 Oxford St., Cambridge, MA 02138, USA.
0301-0104/$ - see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2004.06.040
complex H3N–SO3, whose potential role was first suggested by Lovejoy and Hanson [6] due to its relatively
strong binding energy and extremely low vapor pressure.
Canagaratna et al. [7] have characterized this adduct in
some detail using microwave spectroscopy, and have suggested that clustering may, in fact, increase both its polarity and its stability and, in doing so, enhance its capacity
to act as a nucleation seed. Subsequently, Tao and coworkers [8] explored a variety of species derived from
the combination of NH3 + SO3 + H2O using DFT and
ab initio methods to examine the structures and energetics
of H3N–SO3 H2O (I), H3N H2O–SO3 (II),
HSO3NH2 H2O (III) and H2SO4 NH3 (IV), as well
as their barriers to the interconversion. These workers
concluded that, while H2SO4 NH3 was most stable
and therefore represented the ultimate fate of the SO3/
NH3/H2O system, H3N–SO3 H2O is only 0.5 kcal/
mol higher in energy, and thus, in principle, could also
form in the atmosphere under appropriate conditions.
In further work, the same group used DFT methods to investigate complexes of H3N–SO3 with 1–6, 9 and 12 water
molecules and confirmed the notion that clustering leads
156
S.W. Hunt et al. / Chemical Physics 305 (2004) 155–164
to increased stability of the H3N–SO3 core unit [9]. The
true role of the complex in the atmosphere, however, is
presently unknown.
In this paper, we report the microwave spectroscopic
observation of H3N–SO3 H2O and nine of its isotopically substituted derivatives in a supersonic jet. Previous
work in our laboratory on H3N–SO3 utilized a molecular source in which neat ammonia was injected directly
into a seeded supersonic expansion of SO3 in Ar [7],
but the addition of H2O as a third reactive species in this
system introduces new experimental demands that are
not adequately met with our original nozzle design.
Thus, a new nozzle source allowing rapid mixing of
three reactive species in a pulsed supersonic expansion
is also described. The microwave spectra obtained are
found to be consistent with the structure of the complex
given by Tao and coworkers [8], and a rough estimate of
the nitrogen–sulfur bond length is given.
mic needle into the early portion of a supersonic
expansion of SO3 seeded in Ar. The Ar/SO3 mixture
was prepared by passing approximately 2 atm of argon
over a solid sample of polymerized SO3, while the NH3/
H2O mixture was obtained from the vapor above a 1.0–
1.5 ml sample of 25–30% ammonium hydroxide solution. During the experiment, bubbles could be seen as
the dissolved ammonia was drawn out of solution by
the vacuum, and it became apparent that the concentrations, and thus complex formation, were not constant
over time. Moreover, although the injection needle is designed to minimize mixing of reactants prior to the expansion, there was significant formation of solid
sulfamic acid on its tip, causing additional instability
in experimental conditions and necessitating frequent
cleaning of the nozzle apparatus. As a result, it was necessary to check the signal intensities of known transitions to judge experimental conditions every 10–20
min. Spectra of the parent form of the complex were,
nonetheless, successfully observed in this manner.
For experiments involving isotopic substitution, the
use of an ammonium hydroxide solution, as described
above, was not feasible. In particular, the possibility of
proton exchange within a partially deuterated ammonium hydroxide solution would prohibit the clean production of isotopomers such as D3N–SO3 H2O, diluting
signal intensity and complicating the observed spectra
with numerous partially deuterated derivatives. Thus,
2. Experimental
Spectra were recorded using a Balle–Flygare [10] type
pulsed-nozzle Fourier transform microwave spectrometer, details of which have been described elsewhere
[11]. Complexes were created in situ by allowing the species of interest to combine in a supersonic expansion, the
axis of which is perpendicular to that of the microwave
cavity. For the work described here, initial experiments
employed an injection needle similar to those previously
described [7a,12], in which a mixture of gaseous NH3
and water was introduced via a stainless steel hypoder-
Fig. 1. Schematic diagram of the concentric dual-injection nozzle for
introducing two reactive components into a seeded supersonic expansion. Gas expands out of a 0.8-mm nozzle orifice into a custom built
adaptor that mounts the needle assembly and creates an extended,
flared channel into which the reactive gases are injected. The series 9
pulsed valve was obtained from General Valve Corporation.
S.W. Hunt et al. / Chemical Physics 305 (2004) 155–164
H3N-SO3-D2O
303 ← 202
Arbitrary Intensity
a source design was sought in which both water and ammonia could be independently introduced into the jet.
While a number of geometrical arrangements were tried
for this purpose, the most successful was a concentric
needle design, illustrated in Fig. 1. In this configuration,
one gas is introduced via the inner of two concentric
needles, while the other is admitted through the cylindrical sheath between the inner and outer needles. Using a
0.8 mm nozzle orifice diameter for the supersonic expansion, the size of the outer needle must be less than
0.035500 outer diameter (OD) to avoid excess interference
with the jet and indeed, increasing the OD of the concentric assembly beyond this dimension led to significant
losses in signal intensity. The inner needle, on the other
hand, may range in size from 0.01000 to 0.02200 OD, although the smaller sizes have not been used due to their
potential to clog, particularly with systems that form
solids. As shown in Fig. 1, the inner needle is a piece
of stainless steel tubing about 5.500 long, inserted into a
100 length of 1/1600 OD guide tubing. Both ends of the
guide tube are soldered to the thinner needle tubing to
ensure that gas only flows through the inner tubing
and not the guide piece. For the outer needle, about
3.500 of tubing is inserted into a slightly shorter length
of guide tubing and again the ends are soldered together.
Approximately 0.500 of the outer needle tubing extends
beyond the end of the guide at the bottom. SwagelokTM
nuts are secured to both ends of the inner needle guide
tube and the top end of the outer needle guide. A SwagelokTM tee is used to connect the two pieces with the inner
needle running through the tee and the outer needle below. An additional piece of guide tubing is used to connect the side arm of the tee to the second gas source. Gas
entering the side arm will flow only through the region
between the outer and inner needle tubing. In the region
of the supersonic expansion, the concentric needle assembly is bent into right angle configuration, as shown,
in order to direct the flow of both gases along the axis of
the jet.
As indicated in the figure, for experiments with H3N–
SO3 H2O the best conditions were obtained by flowing
ammonia through the inner and water vapor through
the outer needle. As usual, SO3 was introduced in Ar
through the pulsed valve. This arrangement was also
used to observe spectra of several mixed isotopic species
including H3N–SO3 D2O, D3N–SO3 H2O, and
H3N–SO3 DOH. Although no tests were done to verify that exchange of the hydrogens still did not occur,
significant scrambling would have resulted in an extensive decrease in signal intensity, which was not observed.
Optimal experimental conditions were obtained for
H3N–SO3 H2O using an inner needle with inner diameter of 0.00800 and outer needle with an inner diameter of
0.02300 . The inner needle extended 0.39500 past the final
bend along the expansion axis, while the outer needle extended only 0.36500 , so that the NH3 entered the expan-
157
10371.8
10372
10372.2
10372.4
Frequency (MHz)
Fig. 2. The F = 4 3 component of the 303 202 transition of H3N–
SO3 D2O observed with the concentric dual-injection source. This
spectrum was observed with ammonia and D2O flowing through a
concentric needle, as described in the text. It was collected over 51 s by
averaging 305 gas pulses with 6 free induction decays per pulse. The
linewidth is about 17 kHz, with unresolved deuterium hyperfine
structure evident.
sion 0.0300 beyond the point of entry of the water. Neat
ammonia flowed through the inner needle at a rate of 1–
2 standard cubic centimeters per minute, while the water
vapor was introduced by passing 0.27 atm of argon over
liquid water and through the outer needle. Use of this
source enabled independent optimization of the water
and ammonia flow rates, and resulted in more stable signals that were typically two times larger than those observed using the ammonium hydroxide solution. Known
transitions of H3N–SO3 [7a], H2SO4 [13], and H2O–SO3
[14] were also readily observable with this source.
Isotopically substituted species were observed using
99.9 at.% D2O (Cambridge Isotope Laboratories), a
50/50 mixture of H2O and D2O (for HOD), 94.6 at.%
18
O enriched H218O (Icon Services), 99.5 at.% D enriched ND3 (Icon Services) and 15NH3 synthesized from
99 at.% 15N enriched 15NH4Cl (Icon Services) and KOH
[15]. All 34S species were observed in natural abundance.
Spectral linewidths were typically between 3 and 20 kHz,
presumably broadened by unresolved proton spin–spin
interactions, and deuterium quadrupole hyperfine structure (when present). A sample spectrum of H3N–
SO3 D2O, taken with the concentric needle source is
shown in Fig. 2.
3. Results
Spectral searches were guided by rotational constants obtained from the theoretical structure of
H3N–SO3 H2O [8]. While searching for the J =
2 1 transitions of the parent isotopomer, numerous
158
S.W. Hunt et al. / Chemical Physics 305 (2004) 155–164
lines requiring the simultaneous presence of NH3, SO3
and H2O were observed between 7000 and 7600 MHz.
Subsequent observation of spectra in the J = 3 2 region together with analysis of the 14N hyperfine structure was sufficient to identify a set of a-type transitions
corresponding to a single species. The approximately
symmetric triplet patterns in both the J = 2 1 and
3 2 regions, corresponding to the two Kp = 1 transitions and a central Kp = 0 transition, indicate a nearly
prolate asymmetric top with values of B and C similar
to those predicted. Several b-type transitions were also
located corresponding to an A value about 600 MHz
larger than predicted on the basis of the ab initio structure. Nonetheless, the assignments of these transitions
were confirmed via internally consistent closed energy
loops formed with known transitions in the a-type
spectrum, and by their 14N nuclear hyperfine structure,
which gave quadrupole coupling parameters identical
to those obtained from the a-type transitions.
Transitions were also observed for species dependent on
NH3/SO3/H218O, NH3/34SO3/H2O, NH3/SO3/D2O,
15
NH3/SO3/H2O, NH3/34SO3/H218O, 15NH3/34SO3/
H2O, ND3/SO3/H2O, ND3/SO3/H218O, and a single
NH3/SO3/HOD species. Observation of the ND3 species
was preceded by a short search for the J = 1 0 transitions of D3N–SO3. The three F = 1 1, 2 1 and 0 1
hyperfine components, observed at 7842.749, 7843.244
and 7844.018 MHz, are approximately 20 MHz higher
than predicted from the published H3N–SO3 structure
[7a]. For the complex of interest, hyperfine structure
for the 14N was resolved and analyzed in the singly substituted species containing 14NH3 and in the D2O isotopomer. Deuterium hyperfine structure was not resolved
for any species, and significantly weaker signal intensities
for the 34S/18O, ND3, and ND3/18O substituted forms allowed measurement of only single components, which
were fit as approximate linecenters. Also, due to lower
signal intensities, b-type transitions were only recorded
Table 1
Spectroscopic constants of H3N–SO3 H2O and its isotopic derivativesa
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
eQqaa (MHz)
eQqbb eQqcc (MHz)
Hyperfine components
Rotational transitions
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
eQqaa (MHz)
eQqbb eQqcc (MHz)
Hyperfine components
Rotational transitions
(B + C)/2 (MHz)
(B C)/2 (MHz)
DJ (kHz)
DJK (kHz)
Rotational transitionsc
(B + C)/2 (MHz)
(B C)/2 (MHz)
DJ (kHz)
DJK (kHz)
Rotational transitionsc
a
b
c
H3N–SO3 H2O
H3N–SO3 H218O
H3N–SO3 HOD
4999.7634 (15)
1854.29546 (49)
1821.19415 (35)
0.757 (18)
37.784 (76)
0.4110 (74)
3.4336 (66)
61
15
4999.4185 (21)
1755.46859 (71)
1725.72107 (64)
0.690 (26)
36.04 (18)
0.3711 (86)
3.4528 (78)
45
12
4966.6497 (21)
1817.04531 (77)
1787.58353 (64)
0.786 (26)
35.84 (12)
0.5616 (95)
2.9900 (83)
40
12
H3N–SO3 D2O
H315N–SO3 H2O
H3N–34SO3 H2O
4958 (44)
1743.16730 (92)
1714.48666 (93)
0.571 (71)
33.5 (12)
0.5778 (99)
3.263 (10)
28
7
4947.69965 (99)
1840.51087 (29)
1799.96989 (23)
0.5826 (96)
36.583 (34)
4997.24b
1852.0595 (19)
1819.1113 (27)
0.7568b
37.784b
0.499 (19)
3.4336b
6
2
D3N–SO3 H2O
D3N–SO3 H218O
1787.9663 (14)
61.8b
0.7568b
37.784b
2
1697.6069 (38)
56.0b
0.7568b
37.784b
1
H315N–34SO3 H2O
H3N–34SO3 H218O
1817.9292 (25)
40.6b
0.7568b
37.784b
1
1738.7306 (25)
35.8b
0.7568b
37.784b
1
Numbers in parentheses are one standard error in the least-squares fit.
Held fixed in fit.
Only the strongest hyperfine component was observed.
12
S.W. Hunt et al. / Chemical Physics 305 (2004) 155–164
for the NH3/SO3/H2O, NH3/SO3/H218O, 15NH3/SO3/
H2O, and NH3/SO3/HOD derivatives.
Observed frequencies for each isotopic form were fit
to WatsonÕs A-reduced Hamiltonian for an asymmetric
rotor [16,17], using the program SPFIT [18] to obtain
rotational, centrifugal distortion, and 14N quadrupole
coupling constants (for species with resolved hyperfine
structure). Residuals were generally within the estimated
experimental uncertainties. Due to the scope of the data,
however, the centrifugal distortion constants DK, dJ and
dK were not determinable and were fixed to zero. The A
rotational constant is only well determined for isotopomers in which b-type transitions were observed, but for
the NH3/SO3/D2O species, the dependence of the hyperfine splittings on j = (2B A C)/(A C) roughly determines A. In the other species without b-type transitions,
A was fixed in the fits at a value calculated on the basis
of the predicted isotope shift, which is adequate since the
a-type spectra are rather insensitive to this parameter.
For the NH3/34SO3/H218O, 15NH3/34SO3/H2O, ND3/
SO3/H2O and ND3/SO3/H218O, isotopomers, for which
only K = 0 spectra were observed, only (B + C) is determined from the observed transitions and (B C) was
similarly constrained to the value predicted from the calculated isotope shifts. This should also be suitable, as
for species in which (B C) is measured from the data,
the isotope shifts are typically within 2 MHz of those
predicted (i.e., within about 6%). Moreover, changes in
the fixed values A and (B C) by even as much as 10%
affected the fit rotational constants by less than 0.5 MHz
in all cases. Finally, note that limited data in the weaker
species required fixing distortion constants at values
obtained from the parent. All observed transitions are
listed in the supplementary material and the fitted spectroscopic constants, together with the number of rotational and hyperfine transitions fit for each species, are
given in Table 1.
Several other features of the observed spectrum are
noteworthy. First, the signal intensities of both the atype and b-type transitions optimized at similar levels
of power for the microwave excitation energy, indicating
that la and lb are of similar magnitude. No c-type transitions were identified, although extensive searches were
159
not conducted. Second, only a single form of the HOD
isotopomers was observed. This is consistent with a species containing a hydrogen-bonded water molecule in
which there is a preference for deuterium substitution
in the hydrogen-bonded position. This propensity is
attributable to the reduction in zero point energy associated with substitution by a heavier isotope [19], and indeed the observed species displayed isotopic shifts
clearly indicating deuterium in the hydrogen bonded
position [20]. Finally, all of the assigned transitions occurred as single peaks or as simple 14N nuclear hyperfine
patterns, with no additional observable splittings arising
from tunneling or large amplitude motion of the water.
Such motions are common for complexes of water [21],
and their absence suggests a fairly strong interaction in
the complex.
4. Discussion
The B and C rotational constants reported in Table 1
are in reasonable agreement with those predicted by Tao
and coworkers [8,22] for SO3–NH3 H2O. Assignment
of the spectrum to this complex, however, cannot be rigorously made on this basis alone since a variety of species with similar rotational constants can, in principle,
arise from the combination of SO3, NH3 and H2O. Of
these, H3N–SO3 H2O and H2SO4 NH3 are most
likely in light of their predicted stability [8] and in view
of the known presence of H3N–SO3 and H2SO4 in the
jet. However, in the discussion which follows, we start
with the broader possibility that any of the species (I)–
(IV) calculated by Tao and coworkers could be that observed in this work.
Table 2 summarizes the observed rotational constants as well as those corresponding to the theoretical
structures [8] of H3N–SO3 H2O, HSO3NH2 H2O,
H2O–SO3 NH3 and H2SO4 NH3. Also included in
the table are total dipole moments and their projections
onto the a, b and c inertial axes of the complex calculated using MOLPRO 2000.1 [23] at the MP2/
6-311++G** level of theory. The fitted values of B
and C for the parent form of the observed complex
Table 2
Theoretical constants for parent species in the NH3/SO3/H2O systema
A (MHz)
B (MHz)
C (MHz)
la (D)
lb (D)
lc (D)
ltot (D)
Observed
H3N–SO3 H2O
HSO3NH2 H2O
SO3–H2O NH3
H2SO4 NH3
4999.7634
1854.2955
1821.1942
4400.07
1860.92
1781.50
3.75
3.48
0.36
5.13
4651.25
1942.12
1917.94
3.13
3.34
1.49
4.81
4256.14
1800.87
1704.42
4.98
0.43
1.20
5.14
4803.49
1872.20
1849.90
4.94
1.25
2.06
5.50
a
Rotational constants and dipole moments correspond to the theoretical geometry [8,22] obtained using MOLPRO 2000.1 at the MP2/6311++G** level. la, lb and lc are the components of the total dipole moment along the a, b and c inertial axes, respectively.
160
S.W. Hunt et al. / Chemical Physics 305 (2004) 155–164
Table 3
Predicted isotope shifts in the NH3/SO3/H2O systema,b
Observed
H3N–SO3 H2O
HSO3NH2 H2O
SO3–H2O NH3
H2SO4 NH3
Shift in (B + C)/2 upon isotopic substitution
NH3/SO3/H218O
97.15
93.52
99.71
17.83
NH3/SO3/D2O
108.92
103.11
33.26
NH3/SO3/HOD
35.43
32.36i
73.99j
18.01i
15.72j
15.66k
22.35l
15
NH3/SO3/H2O
NH3/34SO3/H2O
ND3/SO3/H2O
17.50
2.16
49.78
8.04
5.85
47.27
46.55
6.64
161.35
52.48
4.26
187.11
ND3/SO3/H218O
140.14
135.22
175.09
196.11
15
NH3/34SO3/H2O
NH3/34SO3/H218O
19.82
99.01
13.93
99.53
107.11e
82.83f
41.56g,i
15.04h
69.30g,j
8.10
5.30
48.95m
74.10n
143.48o
165.54p
13.38
105.23
11.94c
20.82d
37.85
53.23
24.63
56.86
16.27q
24.85r
Shift in A upon isotopic substitution
NH3/SO3/H218O
0.34
0.43
0.62
144.80
NH3/SO3/D2O
42
45.38
190.39
NH3/SO3/HOD
33.11
26.11
34.78i
160.12j
28.67k
159.23l
15
52.06
82.34
57.32e
72.33f
27.69g,i
43.19h
29.00g,j
92.89
120.39c
198.38d
186.05
1.37
0.10
NH3/SO3/H2O
a
Calculated from geometries [8,22] at the MP2 6-311++G** level.
b
All values in MHz. All shifts are relative to the parent species, H314N–32SO3 H216O.
c
HOSO218OH NH3.
d
H18OSO2OH NH3.
e
HSO3NH2 DOD.
f
DSO3NH2 HOD.
g
HSO3NH2 DOH.
h
DSO3NH2 H2O.
i
D in the hydrogen bond.
j
H in the hydrogen bond.
k
HOSO2OD NH3.
l
DOSO2OH NH3.
m
DSO3ND2 H2O.
n
HSO3ND2 DOH.
o
DSO3ND2 H218O.
p
HSO3ND2 D18OH.
q
HO34SO218OH NH3.
r
H18O34SO2OH NH3.
are seen to lie within 7% of those calculated for all four
species, while the observed value of A is within 15% of
any of the predicted values. This level of agreement
rules out tetramers or higher clusters as the source of
the observed spectra, but does not distinguish between
the above trimers. The 14N hyperfine structure does,
however, unambiguously establish the presence of a single nitrogen atom in the complex, and the observation
of deuterium and 34S substituted species confirms the
presence of hydrogen and sulfur, respectively. These
conclusions are consistent with the observed chemistry,
which require that NH3, SO3 and H2O be simultaneously present in the jet.
Considerably more information is obtained from the
isotope shifts in A and (B + C)/2, which are summarized
in Table 3 and compared with the predicted values for
structures (I)–(IV) at their theoretical geometries. Values of (B C)/2 are not included in the table, as they
are fairly small for all four isomeric forms and are
therefore not particularly informative. Note that for
HSO3 NH2 H2O and H2SO4 NH3, whose formation
requires chemical reaction prior to complexation, the
placement of the rare isotope in the reaction product
(HSO3NH2 or H2SO4) is not unique in all cases. For instance, the incorporation of 18O into the sulfuric acid
formed from SO3 and H218O largely produces HOS-
S.W. Hunt et al. / Chemical Physics 305 (2004) 155–164
O218OH [24], but subsequent complexation with ammonia can place the 18O in either the hydrogen bonded or
free OH positions. Thus, two isotope shifts are included
in the table. Similar ambiguities arise in cases involving
D2O, HOD and ND3, and are also indicated. A number
of less likely substitutions (e.g., incorporation of 18O into SO3) have been omitted. A more complete table of
calculated isotope shifts is given elsewhere [25].
It is apparent from the table that the observed shifts in
the rotational constants are in reasonable agreement with
those predicted for H3N–SO3 H2O, but are in quite
poor agreement with those for either SO3–H2O NH3
or H2SO4 NH3. For example, for the H218O substituted
derivative, the observed shift in (B + C)/2 is 97.15 MHz,
quite close to the calculated value of 93.52 MHz for
H3N–SO3 H2O, but much larger than the 17.83 MHz
value predicted for SO3–H2O NH3, or either of the
values predicted for H2SO4 NH3 (11.94 or 20.82
MHz). Similarly, the observed shift in A (0.34 MHz) is
very close to that predicted for H3N–SO3 H2O (0.43
MHz), but falls far short of that expected for SO3–
H2O NH3 or H2SO4 NH3, both of which are well
over 100 MHz. Similar results are obtained for the other
isotopic forms listed, clearly indicating that neither SO3–
H2O NH3 nor H2SO4 NH3 is the carrier of the
observed spectrum. This conclusion is consistent with
the observation, noted above, that la lb, as the ab intio
calculations of the dipole moments given in Table 2 show
la to be considerably larger than lb for SO3–H2O NH3
and H3N H2SO4, but nearly the same as lb for H3N–
SO3 H2O [26].
The isotopic shift data are much less decisive in
terms of distinguishing between H3N–SO3 H2O and
HSO3NH2 H2O. Indeed, these two complexes differ
only by the position of a proton, with heavy atom distances all quite similar, making a differentiation between the two difficult on the basis of moments of
inertia alone. Moreover, the two complexes cannot
be distinguished on the basis of their dipole moment
components, as la and lb are predicted to be quite
similar for both species. Nonetheless, a number of
other factors do favor assignment of the observed
spectra to H3N–SO3 H2O. According to the calculations of Tao and coworkers [8], NH3-SO3 H2O is 3.1
kcal/mol more stable than HSO3NH2 H2O and the
barrier to interconversion between the two forms is
high (13.5 kcal/mol) [27]. While the addition of water
molecules to form tertiary or larger clusters in the expansion could, in principle, lower this barrier, it seems
doubtful that they would stabilize HSO3NH2 H2O
over H3N–SO3 H2O. Sulfamic acid exists as the
zwitterion both in the gas phase [7,28] and the crystal
[29], and even in the largest of water clusters, i.e.,
bulk solution, the adduct is primarily zwitterionic
[30]. Moreover, previous theoretical calculations indicate a greater stabilization energy for the zwitterion
161
than the neutral form in both polar (e = 40) and
non-polar (e = 2) media [28]. Thus, it seems almost inconceivable that a cluster of any size would favor the
formation of the higher energy product in a free jet
expansion with a nominal temperature of 2 K. Moreover, on the basis of the intensity of its known transitions throughout the experiment, it is certain that a
significant amount of H3N–SO3 was present, and at
the intensity levels observed, it would be surprising
not to have found its water complex in the jet.
HSO3NH2 H2O, on the other hand, is not a species
we would ordinarily have expected to observe, and indeed, as noted above, the possibility was only considered here because it was investigated theoretically in
[8]. To the best of our knowledge, the microwave
spectrum of HSO3NH2 has not been recorded, and
it was therefore not possible to definitively exclude
the possibility of its presence in the jet. However, on
the basis of the above arguments, we assign the observed spectrum to H3N–SO3 H2O, despite the noted
similarity in its theoretical isotope shifts to those of
HSO3 NH2 H2O.
Based on the observed rotational constants for the
complex, we estimate the N–S bond length to be
1.83(13) Å, where the 0.13 Å uncertainty reflects the
range of values obtained in an extensive series of
approximate analyses [25]. A more detailed structure
determination was not possible due, in part, to the significant changes in the geometry of the H3N–SO3 unit
upon complexation. While the constraint of monomer
geometries to their free-molecule values is routine for
the analysis of weakly bound complexes, H3N–SO3 is
a partially bonded unit [7a], whose S–O bond distances
in the trimer are likely different from each other and
from those in free SO3 or H3N–SO3. Such changes are
significant for this system [31] because the H3N–SO3
unit itself contributes measurably to the moments of inertia of the complex. This, together with the absence of
any symmetry elements and the large number of degrees
of freedom (3N 6 = 27), rendered a complete fitting of
the structure impossible, despite the observation of 10
isotopic forms. 18O substitution on the SO3 would presumably aid in locating the oxygen atoms, but was
deemed prohibitively expensive. It is also possible that
the observed rotational constants are effective constants, perturbed by internal motion, and indeed the
large A rotational constant was found to be particularly
problematic in attempting to solve the structure. However, no transitions corresponding to an excited internal
rotor state were positively identified, making quantitative verification difficult. Attempts to extract a more detailed structure for the complex are described elsewhere
[25].
The N–S bond length in H3N–SO3 is 1.957(23) Å
[7a]. Thus, despite a rather large uncertainty, the
1.83(13) Å N–S bond length for H3N–SO3 H2O
162
S.W. Hunt et al. / Chemical Physics 305 (2004) 155–164
quoted above suggests that the addition of an extra water molecule to H3N–SO3 shortens the donor–acceptor
bond by about a tenth of an angstrom. This is in agreement with theoretical results [8] at the MP2 level, using a
6-311++G** basis set, which give bond lengths of 2.123
and 2.007 Å for H3N–SO3 and H3N–SO3 H2O, respectively. Although the theoretical bond lengths are both
somewhat too long, the predicted contraction of 0.12 Å
is in good agreement with experiment. Such a decrease
in bond length is consistent with a body of evidence indicating that near-neighbor interactions exert a strong
influence over the structure of partially bound species
in general [32], and with published studies for this system in particular [7,8,28]. Overall, the level of agreement
between the experimental and theoretical rotational constants and isotope shifts indicates that the calculated
structure [8,22] of H3N–SO3 H2O (I) is at least close
to correct.
It is also of interest to analyze the observed 14N quadrupole coupling constant of the complex, (eQq)aa. In the
absence of electronic rearrangement upon complexation, this parameter is simply the tensor projection of
the free molecule value onto the a-inertial axis of the
trimer, viz.,
ðeQqÞaa ¼ ðeQqÞ0 hP 2 ðcos hÞi
ð1Þ
where (eQq)0 is the quadrupole coupling constant of the
H3N–SO3 moiety along the N–S bond axis, h is the angle
that the N–S bond makes with the a-axis of the complex,
P2(cos h) ” (3cos2h 1)/2, and the angular brackets denote vibrational averaging. Using the known coupling
constant of free H3N–SO3 (1.688 MHz) [7a] and the
observed value of (eQq)aa = 0.411 MHz in H3N–
SO3 H2O, a value of h = 66 is readily obtained if the
vibrational averaging in Eq. (1) is ignored. This result
may be compared with the 56 angle calculated from
the theoretical structure [8] of the complex, providing
what appears to be a reasonable level of agreement.
We note, however, that such a simple projective calculation ignores changes in electronic structure of the
H3N–SO3 upon complexation, which are likely to be significant in light of the large contraction of the N–S
bond. It is instructive, therefore, to estimate the magnitude of these changes, and to investigate the impact they
might have on the structural implications of the measured quadrupole coupling constant.
For H3N–SO3 and (CH3)3N–SO3 [33], Townes and
Dailey analyses [17] of the 14N nuclear quadrupole coupling constants give values of 0.36 and 0.58 electrons, respectively, transferred away from the nitrogen lone pair
upon bond formation to SO3. With nitrogen–sulfur
bond lengths of 1.957 and 1.912 Å, respectively, these
data indicate that in the vicinity of 1.9 Å, the degree
of electron transfer increases at a rate of about 4.9 electrons/Å as the N–S bond length decreases. Thus, using
the theoretical bond shortening of 0.12 Å for H3N–
SO3 H2O relative to that H3N–SO3, we estimate that
about 0.47 electron is transferred from the H3N within
the trimer. With this value, the same equations and assumptions employed previously [7,33] to quantify the
electron transfer in H3N–SO3 and (CH3)3N–SO3, may
be used in a ‘‘reverse Townes and Dailey analysis’’ to
predict a corrected value for (eQq)0 within the complex.
The result is +2.15 MHz, which is quite different from
the 1.688 MHz value of ‘‘free’’ H3N–SO3 because
P2(cosh) crosses zero in this region. If this estimate is
then combined with the observed value of (eQq)aa for
the trimer, an angle of 47 between the N–S bond axis
and the a-inertial axis of the complex is obtained. Interestingly, this result is, again, not far from the 56 corresponding to the computationally derived structure of
Larson and Tao, and indeed, at first glance it seems satisfying that the values of h = 66 (no changes in electronic
structure) and h = 47 (with estimated changes in electronic structure) straddle the theoretical value. The
agreement is deceptive, however, in that the value of
(eQq)0 along the N–S bond axis in the complex which
is required to exactly reproduce the measured projection
onto the a-axis (i.e., 0.411 MHz/P2[cos(56)]) is 13.2
MHz, an entirely unphysical result. In other words,
the observed (eQq)aa for the complex rules out a structure in which h is exactly 56, but does not rule out
structures close to it. This seemingly peculiar situation
arises because the angles in question lie close to, and
on either side of the singularity at 54.7, for which
P2(cos h) = 0.
Finally, as noted above, the observed spectrum appears to arise from a single state, with no evidence of
spectral doubling. Such doubling frequently arises in
weakly bound water complexes due to a tunneling motion that interchanges the H2O hydrogens via rotation
about the C2 axis of the water within the complex [21].
Its absence here suggests that the water is fairly tightly
bound, and indeed, there is precedent for the lack of this
type of motion in other strongly bound water systems
such as (CH3)2HN–H2O [34]. However, other types of
motion such as internal rotation of the SO3 and/or ring
puckering may still be present, even without spectral
manifestation in the ground state. Such motions could
account, at least in part, for an effective A rotational
constant which is 600 MHz larger than expected, but
the available data are insufficient to either prove or disprove this conjecture.
In summary, we have developed a coaxial, dual injection source for seeding several reactive species in a supersonic jet. This source has been used to observe
microwave spectra of the parent and nine isotopically
substituted forms of H3N–SO3 H2O, and a rough determination of the nitrogen–sulfur bond distance has
been made. Further structural refinement is unlikely
without 18O substitution on the SO3, but the experimental data support both previous speculation and subse-
S.W. Hunt et al. / Chemical Physics 305 (2004) 155–164
quent theoretical evidence that the addition of water to
the H3N–SO3 adduct produces a substantial contraction
of the N–S bond. The concomitant increase in stability
may be important to any potential role for this system
in sulfate aerosol nucleation.
Acknowledgements
This work was supported by the National Science
Foundation, the donors of the Petroleum Research
Fund, administered by the American Chemical Society,
and the Minnesota Supercomputing Institute. We are also grateful to Professor Fu-Ming Tao for making the results of [8] available to us prior to publication.
Appendix A. Supplementary material
Supplementary data associated with this article can
be found, in the online version at doi:10.1016/j.chemphys.2004.06.040.
References
[1] B.J. Finlayson-Pitts, J.N. Pitts Jr., Chemistry of the Upper and
Lower Atmosphere, Academic Press, San Diego, 2000.
[2] D.J. Coffman, D.A. Hegg, J. Geophys. Res. 100 (1995) 7147.
[3] R.J. Weber, P.H. McMurry, L. Mauldin, D.J. Tanner, F.L.
Eisele, F.J. Brechtel, S.M. Kreidenweis, G.L. Kok, R.D.
Schillawski, D. Baumgardner, J. Geophys. Res. 103 (1998)
16385.
[4] S.M. Ball, D.R. Hanson, F.L. Eisele, J. Geophys. Res. 104 (1999)
23709.
[5] (a) I. Napari, M. Kulmala, H. Vehkamäki, J. Chem. Phys. 117
(2002) 8418;
(b) I. Napari, M. Noppel, H. Vehkamäki, M. Kulmala, J. Chem.
Phys. 116 (2002) 4221.
[6] E.R. Lovejoy, D.R. Hanson, J. Phys. Chem. 100 (1996)
4459.
[7] (a) M. Canagaratna, J.A. Phillips, H. Goodfriend, K.R. Leopold,
J. Am. Chem. Soc. 118 (1996) 5290;
(b) M. Canagaratna, M.E. Ott, K.R. Leopold, Chem. Phys. Lett.
281 (1997) 63.
[8] L.J. Larson, F.-M. Tao, J. Phys. Chem. A 105 (2001) 4344.
[9] P.M. Pawlowski, S.R. Okimoto, F.-M. Tao, J. Phys. Chem. A 107
(2003) 5327.
[10] T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33.
[11] (a) J.A. Phillips, M. Canagaratna, H. Goodfriend, A. Grushow,
J. Almlöf, K.R. Leopold, J. Am. Chem. Soc. 117 (1995)
12549;
(b)
J.A. Phillips, Ph.D. Thesis, University of Minnesota, 1996.
[12] (a) A.C. Legon, A.L. Wallwork, C.A. Rego, J. Chem. Phys. 92
(1990) 6397;
(b) C.W. Gillies, J.Z. Gillies, R.D. Suenram, F.J. Lovas, E.
Kraka, D. Cremer, J. Am. Chem. Soc. 113 (1991) 2412;
(c) T. Emilsson, T.D. Klots, R.S. Ruoff, H.S. Gutowsky, J.
Chem. Phys. 93 (1990) 6971.
[13] R.L. Kuczkowski, R.D. Suenram, F.J. Lovas, J. Am. Chem. Soc.
103 (1981) 2561.
[14] J.A. Phillips, M. Canagaratna, H. Goodfriend, K.R. Leopold, J.
Phys. Chem. 99 (1995) 501.
[15] D.L. Fiacco, Ph.D. Thesis, University of Minnesota, 2001.
163
[16] J.K.G. Watson, J. Chem. Phys. 46 (1967) 1935.
[17] W. Gordy, R.L. Cook, Microwave Molecular Spectra, Wiley,
New York, 1984.
[18] H.M. Pickett, J. Mol. Spectrosc. 148 (1991) 371.
[19] See, for example H.O. Leung, M.D. Marshall, R.D. Suenram,
F.J. Lovas, J. Chem. Phys. 90 (1989) 700.
[20] The predicted isotope shifts in B and C are a factor of two larger
for substitution of the free hydrogen, so these species would be
clearly distinguishable.
[21] (a) See, for example G.T. Fraser, Int. Rev. Phys. Chem. 10 (1991)
189;
(b) K.R. Leopold, G.T. Fraser, S.E. Novick, W. Klemperer,
Chem. Rev. 94 (1994) 1807.
[22] Larson and Tao did not report rotational constants or dipole
moment components in [8] so we performed our own geometry
optimizations using MOLPRO 2000.1 at the MP2/6-311++G**
level of theory. Our structural and total dipole moment results
are essentially identical to theirs for HSO3NH2 H2O, SO3–
H2O NH3, and H2SO4 NH3, but differ slightly for SO3–
NH3 H2O. The main differences are for the S Owater
distance, which we calculate to be 3.426 Å (compared with
their 3.469 Å), and in the total dipole moment, which we
calculate to be 5.13 D (compared with their 5.29 D).
[23] MOLPRO is a package of ab initio programs written by H.-J.
Werner, P.J. Knowles, with contributions from R.D. Amos, A.
Bernardsson, A. Berning, P. Celani, D.L. Cooper, M.J.O. Deegan,
A.J. Dobbyn, F. Eckert, C. Hampel, G. Hetzer, T. Korona, R.
Lindh, A.W. Lloyd, S.J. McNicholas, F.R. Manby, W. Meyer,
M.E. Mura, A. Nicklass, P. Palmieri, R. Pitzer, G. Rauhut, M.
Schütz, U. Schumann, H. Stoll, A.J. Stone, R. Tarroni, T.
Thorsteinsson.
[24] D.L. Fiacco, S.W. Hunt, K.R. Leopold, J. Am. Chem. Soc. 124
(2002) 4504.
[25] S.W. Hunt, Ph.D. Thesis, University of Minnesota, 2002. This
document is available through UMI Digital Dissertations or upon
request to the authors.
[26] Further evidence that the complex is not H2SO4 NH3 lies in the
observed nitrogen hyperfine constants, which are not close to
those predicted by projecting the NH3 values onto the inertial axes
of the complex. For H3N–SO3 H2O, however, the a-axis of the
complex makes an angle of about 56 with the C3 axis of NH3, and
thus the very small observed value of eQqaa is expected. Another
set of spectra, with hyperfine structure closer to that predicted for
H2SO4–NH3 has been identified but further work is needed to
confirm this assignment.
[27] These values are obtained from the zero point corrected B3LYP
energies of [8].
[28] M.W. Wong, K.B. Wiberg, M.J. Frisch, J. Am. Chem. Soc. 114
(1992) 523.
[29] (a) J.W. Bats, P. Coppens, T.F. Koetzle, Acta Crystallogr. B 33
(1977) 37;
(b) R.L. Sass, Acta Crystallogr. 13 (1960) 320;
(c) F.A. Kanda, A.J. King, J. Am. Chem. Soc. 73 (1951)
2315.
[30] (a) G.A. Benson, W.J. Spillane, Chem. Rev. 80 (1980) 151;
(b) R.L. Benoit, D. Boulet, M. Fréchette, Can. J. Chem. 66 (1988)
3038;
(c) H.P. Hopkins Jr., C.-H. Wu, L.G. Hepler, J. Phys. Chem. 69
(1965) 2244.
[31] For example, a change in bond distance within the SO3 by
0.01 Å (a value near the theoretical increase of this bond),
changes A, B and C by 21, 3 and 0.4 MHz, respectively.
Changes of this magnitude for each of the three distinct SO
bonds, as well as the OSO angles, causes an accumulation of
errors and results in an inability to determine structural
parameters which are not highly correlated with the oxygen
coordinates.
164
S.W. Hunt et al. / Chemical Physics 305 (2004) 155–164
[32] (a) K.R. Leopold, M. Canagaratna, J.A. Phillips, Acc. Chem.
Res. 30 (1997) 57;
(b) K.R. Leopold, in: M. Hargittai, I. Hargittai (Eds.), Advances
in Molecular Structure Research, vol. 2, JAI Press, Greenwich,
1996, p. 103.
[33] D.L. Fiacco, A. Toro, K.R. Leopold, Inorg. Chem. 39 (2000)
37.
[34] M.J. Tubergen, R.L. Kuczkowski, J. Mol. Struct. 352/353 (1995)
335.