The ®rst chromium-53 solid-state nuclear magnetic resonance spectra
of diamagnetic chromium(0) and chromium(VI) compounds
David L. Bryceab and Roderick E. Wasylishen*b
a
b
Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4J3 Canada
Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2G2 Canada.
E-mail: [email protected]
Receivved 17th October 2001, Accepted 17th October2001
First published as an Advvance Article on the web
Chromium-53 is a spin-3=2 nucleus with a relatively small magnetic
moment, low natural abundance, and large quadrupole moment.
These properties have severely hampered the development of 53Cr
NMR, especially in the solid state. In this Communication, the
®rst 53Cr solid-state NMR spectra of prototypal diamagnetic
chromium(0) and chromium(VI) compounds are presented.
Speci®cally, analyses of 53Cr NMR spectra of solid hexacarbonylchromium(0), caesium chromate(VI), and potassium
chromate(VI) have allowed for the determination of 53Cr quadrupolar coupling parameters and the ®rst chromium chemical shift
(CS) tensors. This work demonstrates the potential of 53Cr solidstate NMR, in particular the extreme sensitivity of the 53Cr
quadrupolar coupling constant to the local chromium environment.
Comparisons are made to known 53Cr NMR parameters available
from solution studies, and to the 95Mo solid-state NMR
parameters of analogous molybdenum compounds. The in¯uence
of crystal symmetry present in isomorphic Cr(CO)6 and Mo(CO)6
is strongly re¯ected in the magnitudes of the metal nuclei CS
tensors and in their orientation with respect to their corresponding
electric ®eld gradient tensors.
Chromium is an early transition metal element which continues
to ®nd extensive use in several areas of chemistry1 including
organic synthesis,2 non-linear optics,3 molecular magnetism,4
and superconductivity.5 From a more fundamental perspective,
the nuclear magnetic resonance spectroscopy of chromium-53
(I ˆ 3=2; N.A. ˆ 9.55%; X ˆ 5.64 MHz; Q ˆ ÿ 0.15 by)6 is
conspicuously underdeveloped, especially in the solid state.7,8
Undoubtedly, the paucity of data may be attributed to the
low resonance frequency, low natural abundance, and large
quadrupole moment of this nucleus. Until 1988, only a small
number of compounds had been studied in solution using 53Cr
NMR, e.g., hexacarbonylchromium(0) and some chromate
salts.9±11 More recent solution studies have provided isotropic
chemical shifts and line widths for over ®fty pseudo-octahedral
diamagnetic pentacarbonylchromium-carbene complexes,12
dichromate and halochromate anions, chromyl halides
(CrO2X2), chromyl nitrate, and several complexes of the type
Cr(CO)x(PF3)6 ÿ x .13 The use of supercritical solvents to reduce
53
Cr line widths has been demonstrated recently.14
Solid-state 53Cr NMR studies will provide much more
complete information on the chromium NMR interaction
tensors and their relationship with the local chromium environment. In favorable cases, the complete electric ®eld gradient
(EFG) and chemical shift (CS) tensors, and their relative
orientations, are accessible in principle; however, no such data
y 1 b (barn) ˆ 10ÿ28 m2.
5154
exist to date. In a recent review of progress in solid-state NMR
studies of low-g nuclei, 53Cr is represented by only one out of
the 245 references.15 Previous solid-state 53Cr NMR studies are
very scarce11 and limited to the study of paramagnetic, ferromagnetic, and antiferromagnetic compounds, e.g., CdCr2Se4
and CuCr2S4 ,16 CoCr2S4 ,17 GdCrO3 ,18 ErCrO3 ,19 CrFe,20
and CrH0.93 .21 One NMR study of 51Cr, a radioactive isotope
with no natural abundance, has also been reported.22
To our knowledge, no solid-state 53Cr NMR spectrum of
any diamagnetic compound has ever been reported. In contrast, several 67Zn (I ˆ 5=2) NMR spectra have been reported
(e.g., refs. 15, 23±25); the Larmor frequency, natural abundance, and quadrupole moment of this nucleus are comparable
to those of 53Cr. Recent 95Mo NMR studies of prototypal
molybdenum compounds including hexacarbonylmolybdenum
(0) and sodium molybdate dihydrate have demonstrated the
utility of 95Mo solid-state NMR of powder samples for the
extraction of useful information on the 95Mo EFG and CS
tensors.26 Given the structural similarity between these
molybdenum compounds and their chromium analogues, the
latter should be suitable candidates for exploratory 53Cr solidstate NMR studies.
A systematic investigation of the 53Cr NMR properties of
several prototypal diamagnetic chromium(0) and chromium(VI) compounds has been initiated to probe the 53Cr EFG
and CS tensors and their relationship with the local electronic
structure and molecular symmetry. Results for Cr(CO)6 ,
Cs2CrO4 , and K2CrO4 , are presented in Fig. 1±3 and Table 1.
Chromium-53 NMR spectra were acquired on Bruker AM 300
(7.05 T) and Bruker Avance DMX 400 (9.4 T) spectrometers
with a modi®ed Bruker Z32DR-MAS-7DB probe (7 mm od
rotors). Experiments were carried out at ambient temperature,
and set up using a pulse-acquire sequence on a saturated
solution of Cr(CO)6 dissolved in chloroform. The 53Cr resonance of this solution was set to 0.00 ppm.
(i) Hexacarbonylchromium(0)
The group 6 metal hexacarbonyl compounds, Cr(CO)6 ,
Mo(CO)6 , and W(CO)6 are known to be isomorphous and
crystallize in the space group Pnma.27±30 Old®eld et al. have
presented a 13C and 17O solid-state NMR study of these three
compounds.31 The structures possess a crystallographic mirror
plane in which the central metal atom and two of the carbonyl
moieties lie. For Cr(CO)6 , the chromium±carbon bond lengths
+
vary over a small range, 1.9105 to 1.9185 A, and bond angles
are within 0.9 of the perfect octahedral angles of 90 and
180 .27 Such small, but signi®cant, deviations from perfect
octahedral symmetry in the solid state will in principle result in
a non-zero EFG tensor and asymmetry parameter for the
Phys. Chem. Chem. Phys., 2001, 3, 5154±5157
This journal is # The Owner Societies 2001
DOI: 10.1039/b108295g
Table 1 Summary of
53
Cr NMR parameters and comparison with
95
Mo NMR data for analogous molybdenum compounds
Compound
Nucleus
CQ=MHz
Za
disob=ppm
Oc=ppm
Cr(CO)6
53
Cr
0.348 ‹ 0.008
0.48 ‹ 0.02
ÿ17 ‹ 1
23 ‹ 2
Mo(CO)6
95
Mo
0.091
0.0893 ‹ 0.0002
0.142
0.151 ‹ 0.005
ÿ1854 ‹ 1
ÿ1854 ‹ 1
22 ‹ 2
23 ‹ 0.4
1.17 ‹ 0.01
1.75 ‹ 0.02
1.15
0.00
0.40 ‹ 0.02
0.82
1775 ‹ 3
1765 ‹ 3
8
90 ‹ 10
Cs2CrO4
K2CrO4
Na2MoO4 2H2O
a
53
Cr
Cr
95
Mo
53
g
200
kd
0.00 ‹ 0.05
ÿ 0.09
ÿ 0.026
Ð
Ð
0.7 ‹ 0.1
Euler angles (a,b,g)e
Ref.
100 ‹ 10 , 90 ‹ 5 ,
55 ‹ 7
90 , 90 , 50
90.7 ‹ 0.6 , 90.4 ‹ 0.3 ,
49.6 ‹ 0.8
a,f 40 ‹ 3 , 0 ‹ 10
Ð
Ð
This work
26
35
This work
This work
26
b 95
Z ˆ (VXX ÿ VYY)=VZZ , where jVZZj jVYYj jVXXj.
Mo chemical shifts are with respect to a 2.0 M aqueous solution of Na2MoO4 . c The span
of the chemical shift tensor is de®ned as O ˆ d11 ÿ d33 , where d11 d22 d33 . d The skew of the chemical shift tensor is de®ned as
k ˆ 3(d22 ÿ diso)=O. e Euler angles de®ne the rotations required to bring the electric ®eld gradient tensor frame into coincidence with the principal
axis system of the chemical shift tensor. f The simulated spectrum is insensitive to the value of a. g A conclusive measurement of anisotropy in the
53
Cr chemical shift tensor was not possible.
metal nucleus. The manifestation of these small structural
distortions in solid powdered hexacarbonylchromium(0) is
evident in the central transition portion of its 53Cr magic-angle
spinning (MAS) NMR spectrum (Fig. 1), from which information about the chromium EFG tensor may be extracted. A
typical second-order quadrupolar powder pattern is
observed.32,33 Under MAS conditions, sublimation of the
sample was a signi®cant problem, creating enough pressure to
repeatedly displace the cap from the rotor. A low MAS rate,
500 Hz, was used to minimize sample heating; this was sucient to almost completely concentrate all intensity from the
spinning sidebands into the centreband, as evidenced by the
small di€erence in the simulated spectra presented in Fig. 1(b)
(nrot ˆ 500 Hz) and 1(c) (nrot ˆ 1). Nevertheless, the relative
intensities of the two main singularities of the second-order
powder pattern are most successfully simulated when the ®nite
spinning rate is considered. The 53Cr quadrupolar coupling
constant (CQ) is 348 kHz, a factor of 1.75 less than the result
one obtains by multiplying the known 95Mo quadrupolar
coupling constant for Mo(CO)6 (89.3 kHz)35 by the ratio of
the 53Cr and 95Mo quadrupole moments6 (ÿ0.15 b=ÿ0.022
b ˆ 6.8). Although Mo(CO)6 and Cr(CO)6 are isomorphous,
di€erences in the metal±carbon bond lengths between the two
+
compounds are on the order of 0.15 A; thus, signi®cant
di€erences are to be expected in the magnitude of the components of the EFG tensor at the metal nucleus. Part of
the discrepancy may also arise due to di€erences in the
Sternheimer antishielding factors for the two nuclei.34 The 53Cr
quadrupolar asymmetry parameter, 0.48, is substantially different from the corresponding 95Mo values reported for
Mo(CO)6 from powder26 (0.142) and single-crystal35 (0.151)
95
Mo NMR measurements.
A 53Cr quadrupolar coupling constant of 356 kHz has been
inferred from T1(53Cr) measurements on Cr(CO)6 dissolved in
CDCl3 at 40 C;36 this method relies on the assumption that
the quadrupolar asymmetry parameter, Z, is exactly zero. The
close correspondence between the solution and solid-state 53Cr
quadrupolar coupling constants is fortuitous; the origins of a
non-zero EFG tensor for a formally octahedral molecule in
solution are likely due to intermolecular solvation e€ects
rather than the structural deviations present in the solid.
Although the 53Cr and 95Mo quadrupolar parameters are
considerably di€erent, analysis of the 53Cr NMR spectrum of a
stationary sample of Cr(CO)6 (Fig. 2) indicates that their
respective chemical shift tensors are remarkably similar. The
span of the chromium chemical shift tensor in Cr(CO)6 is 23
ppm, which is identical to the span of the molybdenum chemical shift tensor in Mo(CO)6 .26,35 The principal components
of the chromium chemical shift tensor relative to a saturated
solution of Cr(CO)6 in chloroform are d11 ˆ ÿ 5.5 ppm,
d22 ˆ ÿ 17.0 ppm, and d33 ˆ ÿ 28.5 ppm. Furthermore, the
Fig. 1 (a) Experimental 53Cr MAS NMR spectrum of solid Cr(CO)6
acquired using a pulse-acquire sequence (p=2 ˆ 6.0 ms) with the
transmitter set at 22.576 MHz (B0 ˆ 9.4 T). A recycle delay of 5.0 s was
used to acquire 8549 scans with nrot ˆ 500 Hz. Also shown are the simulated spectra based on the parameters given in Table 1 and (b) using
nrot ˆ 500 Hz and (c) assuming an in®nite MAS rate. Spectra were simulated based on a single chromium site27 using SIMPSON46 (part b)
and WSOLIDS47 (part c).
Fig. 2 (a) Experimental 53Cr NMR spectrum of solid Cr(CO)6 acquired under stationary conditions using a pulse-acquire sequence
(p=2 5.0 ms) with the transmitter set at 16.934 MHz (B0 ˆ 7.05 T). A
recycle delay of 10.0 s was used to acquire 13122 scans (total experiment time 36.5 h). Spectrum (b) is the best-®t simulation (WSOLIDS47), incorporating an anisotropic chemical shift tensor, O ˆ 23
ppm. The simulation shown in (c) is based solely on the quadrupolar
interaction and assumes an isotropic chemical shift tensor.
Phys. Chem. Chem. Phys., 2001, 3, 5154±5157
5155
relative orientations of the metal CS and EFG tensors in the
two hexacarbonyl compounds are identical within error.
Simulation of the spectrum of a stationary sample of Cr(CO)6
shown in Fig. 2(a) is very sensitive to the magnitude and
orientation of the chromium chemical shift tensor. For
example, neglect of anisotropy in the CS tensor results in the
simulated spectrum shown in Fig. 2(c), which is clearly in
disagreement with the experimental result shown in Fig. 2(a).
While it is not possible to determine the absolute orientation of
either the 53Cr EFG or CS tensor based solely on the powder
techniques used here, the absolute orientations of the 95Mo
NMR interaction tensors in the molecular framework are
known for Mo(CO)6 from the single-crystal study of Vosegaard and co-workers.35 Given the isomorphism of Cr(CO)6
and Mo(CO)6 , it is reasonable to conclude that the absolute
orientations of the 53Cr EFG and CS tensors in Cr(CO)6
correspond to those found for 95Mo in Mo(CO)6 , where the
smallest component of the chemical shift tensor (d33) and the
intermediate component of the EFG tensor (VYY)37 lie perpendicular to the mirror plane de®ned by the Pnma space
group symmetry.
Whereas the 53Cr and 95Mo EFG tensors in Cr(CO)6 and
Mo(CO)6 re¯ect the small but signi®cant structural di€erences
between the two compounds (e.g., metal±carbon bond lengths),
the remarkable correspondence of the chromium and molybdenum chemical shift tensors in their respective hexacarbonyl
complexes is a testament to the sensitivity of the CS tensor to
the overall crystal symmetry de®ned by the crystallographic
space group.
(ii) Chromium(VI): Chromate salts
Simulation of the 53Cr NMR spectra of stationary samples of
caesium chromate, Cs2CrO4 (Fig. 3), and potassium chromate,
K2CrO4 (not shown), allows for the extraction of the isotropic
chemical shift and quadrupolar coupling parameters. In
addition, in the case of caesium chromate it was possible to
determine the principal components of the chromium chemical
shift tensor (d11 ˆ 1809.5 ppm, d22 ˆ 1796.0 ppm, and
d33 ˆ 1719.5 ppm relative to Cr(CO)6 in CHCl3) and its
orientation relative to the EFG tensor. Simulation of the
experimental spectrum shown in Fig. 3(a) was not possible
without incorporating an anisotropic chemical shift tensor. We
note that the chemical shift tensors determined for Cr(CO)6
Fig. 3 (a) Experimental 53Cr NMR spectrum of solid Cs2CrO4 acquired under stationary conditions using a quadrupolar echo pulse
sequence (p=2±t1±p±t2±ACQ) with the transmitter set at 22.61743
MHz (B0 ˆ 9.4 T). A recycle delay of 5.0 s was used to acquire 16750
scans (total experiment time 23.3 h). Spectrum (b) is the best-®t simulation (WSOLIDS47), based on a single chromium site,43 taking
into account both quadrupolar and anisotropic chemical shift interactions (Table 1).
5156
Phys. Chem. Chem. Phys., 2001, 3, 5154±5157
and Cs2CrO4 may serve as useful test cases for the calculation
of chromium chemical shifts, and transition metal chemical
shifts in general.38 The isotropic chromium chemical shifts of
chromate salts are known from solution studies to be on the
order of 1800 ppm larger than in chromium(0) complexes such
as Cr(CO)6 .10,11 The same is found in the solid state, with
Cs2CrO4 being 1793 ppm less shielded than Cr(CO)6 . A
similar shift di€erence is found for the analogous molybdenum
compounds, e.g., Cs2MoO4 is 1832 ppm less shielded than
Mo(CO)6 .39
In agreement with 95Mo data on hexacarbonylmolybdenum(0) and sodium molybdate(VI) dihydrate (see Table 1),
the 53Cr quadrupolar coupling constants are several times
larger in the tetrahedral anions than in the octahedral complexes. However, the 53Cr quadrupolar coupling constants in
solid K2CrO4 (1.75 MHz) and Cs2CrO4 (1.17 MHz) di€er
substantially with data derived from T1(53Cr) measurements
carried out in solution. For example, employing a T1 value
extrapolated to in®nite dilution, Haid et al. report CQ(53Cr)
to be 750 kHz for aqueous K2CrO4 .40 The non-zero value in
solution likely arises from solvation e€ects whereas the
principal cause of the large CQ values in the solid state is due to
forced distortion of the chromate tetrahedron in the
crystal lattice. For example, in the solid state it is noted that
upon changing the chromate counterion from caesium to
potassium, an increase in the 53Cr quadrupolar coupling
constant of 50% results. Furthermore, preliminary results on
solid ammonium chromate indicate a quadrupolar coupling
constant on the order of 2.5 MHz, over twice as large as that
for caesium chromate. Consideration of the X-ray structures of
K2CrO441 and (NH4)2CrO442 and the neutron di€raction
structure of Cs2CrO443 suggests that deviation of the O±Cr±O
bond angle from the perfect tetrahedral angle of 109.47 may
be correlated with the observed 53Cr quadrupolar coupling
constants. The largest deviations for each of these compounds are 0.53 (Cs), 0.77 (K), and 2.43 (NH4). The
increasing trend in bond angle deviation, Cs < K < NH4
clearly parallels the observed increase in the 53Cr quadrupolar
coupling constants, 1.17 < 1.75 < 2.5 MHz. Unquestionably,
the 53Cr quadrupolar coupling constant in solid chromate
salts is an extremely sensitive probe of small deviations from
perfect tetrahedral symmetry of the chromate anion.
Finally, it is interesting to note that the data presented in
this Communication have been acquired using standard solidstate NMR methods on 7.05 T and 9.40 T magnets, which
have been commercially available for many years. The question then arises as to why it has taken so long for 53Cr solidstate NMR spectra to be reported for prototypal chromium
compounds. Previous attempts may have been thwarted by an
unfortunate choice of compound or recycle delay. As pointed
out by Smith,15 relaxation times for quadrupolar nuclei with
large quadrupole moments are not guaranteed to be short
(e.g, milliseconds or less); recycle delays on the order of several seconds have indeed been necessary in the case of 53Cr
and 95Mo. For example, the T1(95Mo) in hexacarbonylmolybdenum(0) is 15.9 s.35 Applications of 53Cr NMR are limited to
relatively symmetric species due to the large quadrupole
moment of this nucleus; however, the advent of `` ultra-high ''
®eld spectrometers, e.g., 18.8 T and 21.1 T, will result in much
improved 53Cr NMR sensitivity, thereby allowing for the
study of a wider range of compounds. Cross-polarization from
1
H in compounds where protons are present will undoubtedly
bene®t future 53Cr solid-state NMR studies; CP to 95Mo has
already been shown to be bene®cial.44 Lipton et al. have
recently presented a general strategy for the NMR observation
of spin-n=2 quadrupolar nuclei in dilute environments45 which
will certainly be valuable in future 53Cr solid-state NMR
studies. Work is currently underway in our laboratory to
realize these bene®ts and to further the development of 53Cr
solid-state NMR.
Acknowledgements
The authors thank all members of the solid-state NMR group
at the University of Alberta, Dr Klaus Eichele, and Prof.
Glenn Penner for valuable comments. We thank Dr Tom
Nakashima and Dr Glen Bigam for technical assistance. All
9.4 T spectra were obtained in the Bio-NMR centre at the
University of Calgary. We thank Prof. Hans Vogel, Dr Robert
Cook, and Dr Deane McIntyre at the University of Calgary
for spectrometer time, technical assistance, and hospitality.
R. E. W. is a Canada Research Chair in Physical Chemistry at
the University of Alberta. This research is supported by the
Natural Sciences and Engineering Research Council of
Canada (NSERC). D. L. B. thanks NSERC, the Izaak Walton
Killam Trust, the Walter C. Sumner Foundation, and Dalhousie University for postgraduate scholarships.
References
1 D. A. House, Adv. Inorg. Chem., 1997, 44, 341.
2 (a) C. Bolm and K. Muiz, Chem. Soc. Rev., 1999, 28, 51; (b) M.
Avalos, R. Babiano, P. Cintas, J. L. JimeÂnez and J. C. Palacios,
Chem. Soc. Rev., 1999, 28, 169; (b) K. H. DoÈtz and P. Tomuschat,
Chem. Soc. Rev., 1999, 28, 187; (d) L. S. Hegedus, Acc. Chem. Res.,
1995, 28, 299.
3 I. R. Whittall, A. M. McDonagh, M. G. Humphrey and
M. Samoc, Adv. Organomet. Chem., 1998, 42, 291.
4 (a) T. Mallah, S. ThieÂbaut, M. Verdaguer and P. Veillet, Science,
1993, 262, 1554; (b) S. Ferlay, T. Mallah, R. OuaheÁs, P. Veillet
and M. Verdaguer, Nature, 1995, 378, 701; (c) O. Sato, T. Iyoda,
A. Fujishima and K. Hashimoto, Science, 1996, 271, 49.
5 (a) D. C. H. Snowling and C. R. M. Grovenor, J. Mater. Chem.,
1993, 3, 473; (b) C. Martin, M. Hervieu, A. Maignan, C. Michel,
B. Raveau, M. Daturi and F. LetouzeÂ, Physica C, 1997, 292, 32;
(c) F. LetouzeÂ, C. Martin, C. Michel, M. Hervieu, A. Maignan, R.
Seshadri and B. Raveau, Eur. Phys. J.: Appl. Phys., 1998, 1, 285.
6 (a) W. Ertmer, U. Johann and R. Mosmann, Z. Phys. A, 1982,
309, 1; (b) P. PyykkoÈ, Mol. Phys., 2001, 99, 1617.
7 M. Minelli, J. H. Enemark, R. T. C. Brownlee, M. J. O'Connor
and A. G. Wedd, Coord. Chem. Rev., 1985, 68, 169.
8 P. Granger, in T he Encyclopedia of Nuclear Magnetic Resonance
ed. D. M. Grant and R. K. Harris, Wiley Inc., Chichester, UK,
1996 , pp. 38893900..
9 Y. Egozy and A. Loewenstein, J. Magn. Reson., 1969, 1, 494.
10 B. W. Epperlein, H. KruÈger, O. Lutz, A. Nolle and W. Mayr,
Z. Naturforsch. A, 1975, 30, 1237.
11 See: C. Brevard, P. S. Pregosin and R. Thouvenot, in T ransition
Metal Nuclear Magnetic Resonance, Studies in Inorganic Chemistry, ed. P. S. Pregosin, Elsevier, Amsterdam, 1991, Vol. 13, and
references therein.
12 A. Hafner, L. S. Hegedus, G. deWeck, B. Hawkins and
K. H. DoÈtz, J. Am. Chem. Soc., 1988, 110, 8413.
13 M. F. A. Dove, E. M. Lloyd Jones and R. J. Clark, Magn. Reson.
Chem., 1989, 27, 973.
14 S. Gaemers, J. Groenevelt and C. J. Elsevier, Eur. J. Inorg. Chem.,
2001, 829.
15 M. E. Smith, Annu. Rep. NMR. Spectrosc., 2000, 43, 121.
16 (a) G. N. Abelyashev, V. N. Berzhanskij, N. A. Sergeev and Yu.
V. Fedotov, Phys. L ett. A, 1988, 133, 263; (b) G. N. Abelyashev,
V. N. Berzhansky, S. N. Polulyakh and N. A. Sergeev, Physica B,
2000, 292, 323.
17 V. N. Derkachenko, N. M. Kovtun, V. K. Proporenko, A. F.
Safarov, A. A. Shemyakov and E. A. Eivazov, Izv. Akad. Nauk
SSR, Neorg. Mater., 1984, 20, 1270.
18 A. S. Karnachev, Yu. I. Klechin, N. M. Kovtun, A. S. Moskvin,
E. E. Solov'ev and A. A. Tkachenko, Zh. Eksp. Teor. Fiz., 1983,
85, 670. (English translation: A. S. Karnachev, Yu. I. Klechin,
N. M. Kovtun, A. S. Moskvin, E. E. Solov'ev and A. A. Tkachenko, Sov. Phys. JETP 1983, 58, 390.)
19 A. S. Karnachev, N. M. Kovtun, M. I. Kurkin, E. E. Solov'ev
and A. A. Tkachenko, Zh. Eksp. Teor. Fiz., 1983, 85, 224.
(English translation: A. S. Karnachev, N. M. Kovtun, M. I.
Kurkin, E. E. Solov'ev and A. A. Tkachenko, Sov. Phys. JET P,
1983, 58, 130).
20 S. M. Dubiel and H. LuÈtgemeier, Phys L ett. A, 1981, 84, 396. and
references therein
21 J. K. Poniak, B. Nowak and M. Tkacz, Z. Phys. B, 1997, 104, 255.
22 J. Boysen, A. Kettschau and W. D. Brewer, Phys. L ett. A, 1983,
95, 502.
23 (a) G. Wu, Chem. Phys. L ett., 1998, 298, 375; (b) S. Sham and G.
Wu, Can. J. Chem., 1999, 77, 1782.
24 (a) F. H. Larsen, A. S. Lipton, H. J. Jakobsen, N. Chr. Nielsen
and P. D. Ellis, J. Am. Chem. Soc., 1999, 121, 3783; (b) A. S.
Lipton, G. W. Buchko, J. A. Sears, M. A. Kennedy and P. D.
Ellis, J. Am. Chem. Soc., 2001, 123, 992.
25 T. Vosegaard, U. Andersen and H. J. Jakobsen, J. Am. Chem.
Soc., 1999, 121, 1970.
26 K. Eichele, R. E. Wasylishen and J. H. Nelson, J. Phys. Chem., A,
1997, 101, 5463.
27 (a) A. Jost, B. Rees and W. B. Yelon, Acta Crystallogr., Sect. B,
1975, 31, 2649; (b) B. Rees and A. Mitschler, J. Am. Chem. Soc.,
1976, 98, 7918.
28 T. C. W. Mak, Z. Kristallogr., 1984, 166, 277.
29 D. M. Adams and I. D. Taylor, J. Chem. Soc., Faraday T rans. 2,
1982, 78, 1051.
30 W. RuÈdor€ and U. Hofmann, Z. Phys. Chem. B, 1935, 28, 351.
Although an incorrect space group was assigned, this early
work establishes the isomorphism of the group 6 metal hexacarbonyls.
31 E. Old®eld, M. A. Keniry, S. Shinoda, S. Schramm, T. L.
Brown and H. S. Gutowsky, J. Chem. Soc., Chem. Commun.,
1985, 791.
32 (a) E. Kundla, A. Samoson and E. Lippmaa, Chem. Phys. L ett.,
1981, 83, 229; (b) A. Samoson, E. Kundla and E. Lippmaa, J.
Magn. Reson., 1982, 49, 350.
33 L. Frydman, Annu. Rev. Phys. Chem., 2001, 52, 463.
34 (a) R. M. Sternheimer, Phys. Rev., 1954, 95, 736; (b) R. M.
Sternheimer, Phys. Rev., 1966, 146, 140.
35 T. Vosegaard, J. Skibsted and H. J. Jakobsen, J. Phys. Chem., A,
1999, 103, 9144.
36 R. T. C. Brownlee and B. P. Shehan, J. Magn. Reson., 1986, 66,
503.
37 Note that we employ the conventions used in ref. 26.
38 (a) R. Bouten, E. J. Baerends, E. van Lenthe, L. Visscher, G.
Schreckenbach and T. Ziegler, J. Phys. Chem., A, 2000, 104, 5600;
(b) G. Schreckenbach and T. Ziegler, Int. J. Quantum Chem., 1997,
61, 899.
39 V. M. Mastikhin, O. B. Lapina and R. I. Maximovskaya, Chem.
Phys. L ett., 1988, 148, 413.
40 E. Haid, D. KoÈhnlein, G. KoÈssler, O. Lutz and W. Schich,
J. Magn. Reson., 1983, 55, 145.
41 J. A. McGinnety, Acta Crystallogr., Sect. B, 1972, 28, 2845.
42 B. M. Gatehouse and P. Leverett, J. Chem. Soc. A, 1969, 1857.
43 A. J. Morris, C. H. L. Kennard, F. H. Moore, G. Smith and H.
Montgomery, Cryst. Struct. Commun., 1981, 10, 529.
44 J. C. Edwards and P. D. Ellis, Magn. Reson. Chem., 1990, 28, 529.
45 A. S. Lipton, J. A. Sears and P. D. Ellis, J. Magn. Reson., 2001,
151, 48.
46 M. Bak, J. T. Rasmussen and N. Chr. Nielsen, J. Magn. Reson.,
2000, 147, 296.
47 K. Eichele and R. E. Wasylishen, WSOLIDS NMR Simulation
Package, Version 1.17.26, 2000.
Phys. Chem. Chem. Phys., 2001, 3, 5154±5157
5157