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Phil. Trans. R. Soc. A (2012) 370, 5225–5235
doi:10.1098/rsta.2012.0088
To be or not to be: the early history of
H3 and H+
3
BY HELGE KRAGH*
Centre for Science Studies, Department of Physics and Astronomy,
Aarhus University, 8000 Aarhus, Denmark
Triatomic hydrogen became a major research area only after 1980, but its history goes
back to J. J. Thomson’s discovery in 1911. In fact, the possible existence of H3 was
suggested as early as 1895. This paper outlines the history of H3 and H+
3 up to the mid1930s, when chemists and physicists ceased to believe in the existence of the H3 molecule.
In the intervening years, there was a great deal of interest in ‘active hydrogen’ and also in
the configuration of H3 , which was examined by Bohr in 1919. While H3 was abandoned,
+
H+
3 was not. Although the properties of H3 were largely unknown, the existence of the
ion was firmly established, and its structure studied by means of the new methods of
quantum chemistry.
Keywords: triatomic hydrogen; active hydrogen; J. J. Thomson; N. Bohr; G. Wendt
1. Triatomic hydrogen in 1895
The triatomic hydrogen ion H+
3 grew out of J. J. Thomson’s experiments
on positive rays emitted in discharge tubes. These rays, also known as canal
rays, were discovered by the German physicist Eugen Goldstein in 1886 and
subsequently investigated in great detail by his compatriot Wilhelm Wien
[1]. A leader in the study of Kanalstrahlen (and a Nobel laureate of 1912),
Wien accepted Thomson’s discovery not only of H+
3 but also of the neutral
H3 molecule.
The generally held belief that the stories of H+
3 and H3 have their beginnings
in Thomson’s experiments is not quite justified. More than 15 years earlier, one
can find in the chemical literature references to the possible existence of H3 ,
which a few scientists thought might represent the newly discovered helium gas.
Originally hypothesized to be a solar element, in 1895, He was unexpectedly
found in terrestrial minerals [2]. The peculiar double structure of its spectrum
seemed to indicate that there were two species of He: the ordinary one (with
an atomic weight of about 5) and ‘parhelium’ (with an atomic weight of 3) [3].
The recognized Czech chemist Bohuslav Brauner thought that parhelium might
be an allotropic form of hydrogen, H3 , that is structurally similar to ozone, O3
[4,5]. His hypothesis was short-lived and considered only by a few scientists. With
*[email protected]
One contribution of 21 to a Theo Murphy Meeting Issue ‘Chemistry, astronomy and physics of H+
3 ’.
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H. Kragh
the recognition that there is only one kind of helium, albeit with two different
systems of spectral series, Brauner’s hypothesis was forgotten. There is no reason
to believe that Thomson knew of it.
2. J. J. Thomson: experiments and interpretations
By the early 1910s, Joseph John Thomson was Britain’s most distinguished
physicist. Following his discovery of the electron in 1897, he developed an
ambitious theory of the atom, which, for nearly a decade, was considered the best
candidate for the internal structure of atoms [6]. However, the original Thomson
model soon ran into trouble, primarily because it was realized that the number
of electrons in atoms was only of the order of the atomic weight. This forced him
to take the positive charge more seriously than he had done previously (when
he thought of the positive charge as a massless fluid smeared out over the entire
volume of the atom). It was in this context that he embarked on an extensive
experimental research programme with the aim of investigating positive rays in
discharge tubes, a line of research he cultivated from about 1908 to 1914 [7,8].
During his experiments with electric and magnetic deflections of positive rays, he
noted an ‘X3 ’ component that had a mass-to-charge ratio three times that of the
hydrogen atomic ion,
m m =3
.
e X3
e H+
The result might have been due to a C4+ ion, but Thomson found it more likely
that the component was H+
3 , a hypothesis he first stated in the February 1911
issue of Philosophical Magazine [9]. Over the next few years, he substantiated
the hypothesis, which he presented at various scientific meetings and described
in detail in a monograph of 1913, Rays of Positive Electricity [7].
According to Thomson, not only had he discovered a new molecular ion, H+
3 , he
had also discovered the neutral molecule H3 . Although he was unable to obtain
a spectrum of the new and elusive gas, he was convinced that it existed in a
stable form. Remarkably, it appeared to be almost chemically inert and resistant
to decomposition by electric discharges. ‘X3 seems more stable than any known
allotropic form of an element’, he pointed out [7, p. 122]. Although Thomson
abandoned his research programme on positive rays in 1914, 20 years later he
returned to it, repeating his belief in a permanent form of H3 . Looking back
on his early experiments, the 78 year old physicist wrote that the H3 particles
occurred with great regularity when certain solids such as KOH were bombarded
by cathode rays. ‘The H3 I obtained from this source was a permanent gas’, he
stated [10, p. 1026].
Well aware that a triatomic hydrogen molecule conflicted with ordinary ideas of
valence, Thomson nonetheless suggested that a triangular form of the molecule
might agree with modern theories of atomic structure. After all, in the early
1910s, the chemical theory of valence was empirical and lacked support from
atomic theory. Using his favoured name ‘corpuscle’ for the electron, in 1913 he
suggested to ‘regard an atom of hydrogen as consisting of a positive nucleus and
one negative corpuscle’, in which case he saw ‘no reason why a group of three
of these arranged so that their axes form a closed ring should not form a stable
arrangement’ [7, p. 121]. The quotation might indicate that Thomson at that
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Early history of H3 and H3+
5227
time accepted Ernest Rutherford’s nuclear atom, but this was not the case. The
‘positive nucleus’ did not refer to Rutherford’s tiny central charge, but to the inner
part of the atom, which he conceived as an arrangement of concentric layers of
positive and negative electrons governed by attractive and repulsive forces, some
of them known and others unknown.
3. Theoretical intermezzo
Niels Bohr stayed with Thomson as a postdoctoral fellow in Cambridge, UK, from
September 1911 to March 1912, after which he moved on to Manchester, UK, to
work under Rutherford (who had himself been with Thomson as a postdoctoral
fellow). Shortly after having published his seminal paper on atomic theory in the
summer of 1913, Bohr attended the meeting of the British Association for the
Advancement of Science in Birmingham, UK. In the discussion session following
a talk Thomson gave on his X3 = H3 hypothesis, the young Dane suggested as
an alternative that X3 might be a super-heavy isotope of hydrogen rather than a
molecule. (At that time, the name ‘isotope’ had not yet been coined.) Nearly 50
years later, Bohr recalled: ‘I just took up the question of whether in hydrogen one
could have what you now call tritium. And then I saw that it was a way to show
this by its diffusion in palladium. Hydrogen and tritium will behave similarly
but the masses are so different that they will get separated out’ (interview
with N. Bohr of 1 November 1962, American Institute of Physics transcript;
http://www.aip.org/history/ohilist/4517_1.html).
Bohr’s suggestion was not taken seriously, but, after having returned to
Copenhagen, Denmark, he continued to think about it, realizing that in principle
the question might be resolved by spectroscopic precision experiments. According
to his theory of one-electron atoms, Rydberg’s constant R would vary with the
reduced mass and the wavelengths thus depend on the mass of the nucleus. In
the case of tritium (T) and ordinary hydrogen (H), the difference in wavelength
would be
RH ∼
Dl = lH 1 −
= 3.6 × 10−5 lH ,
RT
which Bohr thought was ‘of an order open for detection’. However, experiments
made in Copenhagen failed to disclose the isotope effect, which was established
only for hydrogen isotopes when deuterium was discovered in 1932.
In 1919, in a little known paper published by the Nobel Institute in Stockholm,
Sweden [11], Bohr examined theoretically the H3 molecule, which he assumed
consisted of three electrons rotating in a circular orbit around the nucleus in
the middle of the system (figure 1). Having concluded that the system was
mechanically stable, he suggested a mechanism that would allow an ionized H2
gas to form H3 in spite of the direct process 3H2 → 2H3 being endothermic.
His theoretical investigation focused on H3 rather than on H+
3 , the reason being
that for the latter system ‘no configuration of mechanical equilibrium . . . can be
formed’. If an electron were removed from H3 , the ion would split up according to
+
H+
3 → H2 + H .
On the other hand, Bohr’s calculations showed that H3 could add one more
electron and form a stable H−
3 ion, which he predicted ‘may exist permanently
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5228
H. Kragh
(a)
(b)
a
a
b
c
b
Figure 1. Bohr’s 1919 models of H−
3 (a) and H3 (b) [11].
HI
HIII
HII
Figure 2. Stark’s view of triatomic hydrogen [12].
in the absence of external agencies’. Of course, his theoretical results ran
−
counter to Thomson’s experimental detection of H+
3 (and the absence of H3 ),
but Bohr thought that the disagreement might only be apparent: perhaps the
decay of the H+
3 ions was somehow delayed under the conditions governing
Thomson’s experiments.
Bohr did not follow up his paper of 1919, which was not well known and
had little effect on later research. Neither did other atomic physicists in the
tradition of the old Bohr–Sommerfeld quantum theory feel tempted to analyse the
hypothetical molecule. The only prominent physicist, apart from Thomson and
Bohr, who supported the existence of H3 was Johannes Stark in Germany [12]. In
1913—the same year he discovered the spectroscopic Stark effect—he proposed a
triangular model of H3 similar to the one suggested by Thomson (figure 2). Stark
also endorsed the claim of H+
3 , but his work in the area of hydrogen molecules
and ions had even less impact than Bohr’s.
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Early history of H3 and H3+
8
H+3
7
electrometer current
5229
6
5
4
3
H+2
2
1
0
H+3–1
H+2–1
0.5
H+
1.0
2.0
1.5
m/e µ 1/(V2+V3)
2.5
3.0
Figure 3. Experiment of 1925 showing the existence of the H+
3 ion [15].
4. H+
3 vindicated
Whereas the question of neutral H3 evolved into a muddled controversy, the
existence of the positive ion was never seriously contested. Not only were Thomson’s experiments and interpretations widely accepted, evidence in support of H+
3
was also provided by many later researchers. Thus, in experiments with positive
hydrogen rays of 1916, the Canadian–American physicist Arthur Dempster
+
+
confirmed the presence of H+
3 in addition to the known ions H and H2 . Contrary
+
to Thomson, Dempster did not regard either H3 or H3 as a stable gas, ‘since
it is not present when there is no dissociation of the hydrogen molecules’ [13].
Whereas, in one of his papers of 1916 [14], Dempster suggested the formation
process H+ + H2 → H+
3 , in another [13] he concluded as follows: ‘Electrons . . .
ionize hydrogen by detaching a single elementary charge from the molecule. They
are not able to dissociate the gas. The positive molecules so formed are able to
dissociate the gas. When this occurs, the complex H+
3 is formed.’
American and German physicists applied improved versions of Dempster’s
ionization method to extend his results and suggest mechanisms for the
production of H+
3 (figure 3). In 1924, Thorfin R. Hogness and E. G. Lunn, two
chemists at the University of California, suggested that H+
3 was produced in a
secondary process, H+ + H2 → H+
,
the
primary
process
being
H 2 → H + H+ + e −
3
[16]. The following year they retracted their suggestion, now concluding that ‘the
+
+
+
formation of H+
2 is the primary process, and . . . H and H3 are formed from H2
as the result of secondary collisions with gas molecules’ [17]. They assumed H+
2
+
to be metastable, colliding with a H2 molecule according to H2 + H+
2 → H3 + H.
This was the same process that Dempster had proposed in 1916, if less detailed
+
and in words only. Hogness and Lunn also considered the reaction H+
2 →H+H
followed by H+ + H2 → H+
3 , but argued that these processes occurred much less
frequently. In a slightly earlier paper, the Princeton physicist Henry DeWolf
Smyth [15] had also discussed the formation of H+
3 , including the possibilities
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5230
H. Kragh
50
H+3
electrometer current
40
H2O+
30
20
10
H+
0.06
HD2+
D+(+H2+)
0.08
0.10
0.12 0.14
magnet current (A)
D2+
0.16
Figure 4. Mass spectrum of 1935, showing ions in a hydrogen mixture with 16% deuterium [20].
+
+
+
H+ + H2 → H+
3 and H + H2 → H3 . However, he did not suggest the main
reaction proposed a few months later by Hogness and Lunn, which was confirmed
by Gaylord Harnwell and other researchers [18].
A most convincing demonstration of H+
3 was produced when Kenneth
Bainbridge at the Franklin Institute, Swarthmore, PA, and later Overton Luhr
at Union College, Schenectady, NY, detected the ion in the mass spectrum
of hydrogen isotopes [19,20]. One of Luhr’s spectra is shown in figure 4. The
spectroscopic discovery of deuterium gave rise to some new research on H+
3 . Two
physical chemists in Berlin, Germany, Hartmut Kallmann and W. Lasareff, looked
+
in vain for a hydrogen compound of mass 4, supposed to be D+
2 or H2 D , but they
+
succeeded in detecting HD in their mass spectrograph [21,22]. They concluded
that H2 D+ must be extremely rare compared with H+
3 . However, Bainbridge found
lines in his mass-spectrographic measurements that corresponded to a mass of
4.0285 and which he attributed to H2 D+ [23]. In experiments from 1933, he
+
reported the detection of more hydrogen molecular ions, including H2+
3 , D2 H
and D+
3 [24].
In spite of the generally held belief in H+
3 , the situation in the 1920s was not
entirely satisfactory: although the ion turned up as an experimental signal, almost
nothing was known of its properties or molecular constitution. Several groups of
scientists looked for the spectrum of H+
3 , yet nothing came out of their efforts. In
view of the much later development, it is of interest to recall that young Gerhard
Herzberg, at the time a doctoral student at Darmstadt Technische Hochschule in
Germany, dealt with the subject in his dissertation of 1927 [25]. A study of the
so-called secondary spectrum of hydrogen led Smyth and his collaborator Charles
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Early history of H3 and H3+
5231
Brasefield [26] to the tentative conclusion that they had found spectroscopic
evidence for H+
3 , but the evidence was indirect and not generally considered
convincing. Although the triatomic hydrogen ion was accepted by the early 1930s,
it remained an open question what it was, more precisely.
If experiments could not yield an answer, perhaps theory could. This, at
any rate, was the philosophy of the first-generation quantum chemists who, in
the 1930s, applied their computational methods to simple molecules such as H2
and H+
3 . Thus, Charles Coulson at the University of Cambridge concluded that,
while H+
3 in its ground state existed in the form of an equilateral triangle, the
excited states would be unstable [27]. Also according to Joseph Hirschfelder, a
physical chemist at the University of Wisconsin, the triangular structure was more
stable than the linear structure. He concluded after lengthy calculations that the
H+
3 ion had a stable configuration corresponding to a separation between the
nuclei of about 1.79 Å with the nuclei lying intermediate between a right and an
+
equilateral triangle [28]. For the reaction H2 + H+
2 → H3 + H, he found that it was
−1
exothermic by 40 kcal mol . Moreover, he estimated the vibration frequencies of
the equilateral form of H+
3 , finding that ‘the z and y modes of vibration should
be infra-red and therefore susceptible to direct experimental observation’. The
remark was potentially important, but it was not followed up and did not lead
to an experimental search for the spectral lines.
5. A chemical enigma
The question of the neutral H3 molecule turned out to be much more complex and
controversial than Thomson had anticipated [29]. Following a work by William
Duane and Gerald Wendt in 1917 [30], over the next decade several chemists and
physicists claimed to have produced evidence for a new reactive form of hydrogen
that most likely was H3 . Francis Aston, inventor of the mass spectrograph and
a former assistant to Thomson, was among those who supported the case of the
H3 molecule, the mass of which he determined to be 3.025 atomic mass units
(compared with H = 1.008). In the second edition of his book Isotopes, he wrote
about triatomic hydrogen: ‘The mass deduced proves in a conclusive manner
that the particle causing it [the mass-3 parabola] is a molecule of three hydrogen
atoms’ [31, p. 70].
The Duane–Wendt paper of 1917 introduced effectively the hypothesis of
‘active hydrogen’, a polyatomic form with unusual properties that, in most cases,
was identified with H3 . According to the two Americans, the new gas was probably
a hydrogenic version of ozone, a proposal Wendt elaborated on in a later paper
written with his student Robert Landauer [32]:
the properties of the new gas are precisely those to be expected of an ozone form. . . .
Its boiling point is much higher than that of ordinary hydrogen. And it is unstable, for
it cannot be detected if more than about a minute is allowed to elapse between the time of
its formation and its reaction with a reducible substance. All the evidence obtained, then,
points to the formation of triatomic hydrogen, perhaps properly called ‘hyzone’, when ever
hydrogen is ionized.
The name ‘hyzone’ or the German equivalent ‘Hyzon’ never caught on. It was
occasionally used in the 1920s and 1930s, but today it is nothing but a historical
curiosity [33].
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5232
H. Kragh
Wendt and most other protagonists of H3 agreed that the new gas was unstable,
and perhaps very much so. Many researchers estimated its lifetime to be of the
order of 1 min, but there were also suggestions of the much shorter lifetime 3 ×
10−8 s, the same order as the lifetime of ordinary excited molecules [34]. As to the
experimental evidence cited in favour of H3 , the following were widely discussed:
— Under the action of alpha rays on pure hydrogen, the volume of the gas
contracted. This might indicate a partial transformation of diatomic to
triatomic hydrogen: 3H2 → 2H3 .
— The boiling point of active hydrogen was much higher (approx. 70 K) than
ordinary hydrogen.
— The new gas combined readily with nitrogen to form ammonia. A possible
reaction mechanism was two successive collisions of the N2 molecule with
H3 groups, (N2 + H3 ) + H3 → 2NH3 , such as proposed by the Canadian
chemist Audrey Grubb [35].
— Active hydrogen formed H2 S when passing over unheated sulphur powder.
By the early 1920s, the new H3 gas seemed to be on its way to recognition, but
10 years later, the situation had changed. Grubb and a few others maintained
faith in the existence of H3 , and as late as 1935 he provided ‘new evidence for
the existence of an active form of hydrogen that differs markedly in its properties
from atomic hydrogen’ [36]. He suggested that the best candidate for the active
form was H3 . To my knowledge, this was the last time the H3 hypothesis was
defended in the scientific literature until it was revived much later.
6. ‘Triatomic hydrogen does not exist’
From about 1925, an increasing number of chemists and physicists questioned
the existence of H3 in the form of active hydrogen. Together with the missing
spectroscopic evidence, the accumulation of experimental arguments against the
elusive molecule had the effect that neutral triatomic hydrogen lost its credibility.
It was downgraded from ‘likely’ to ‘most unlikely’.
What can reasonably be called the campaign against H3 started in 1925
with a series of objections raised by the Russian chemist Abraham Bach, and
a few years later they were reinforced by Fritz Paneth in Austria, Harold
Urey in the USA, and several other chemists [29]. Paneth concluded that the
effects ascribed to H3 could in almost all cases be explained by the presence
of traces of H2 S in the ‘active’ hydrogen gas [37]. According to Urey and
his student Hugh Smallness, many of the experiments that had been used
in support of H3 were poor and could not even be reproduced [38]. For
example, careful dilatometric measurements failed to produce a contraction
effect comparable to the one reported by Wendt and his collaborators. Other
precision experiments indicated that the formation of small amounts of NH3 and
H2 S could be explained without assuming any formation of active hydrogen. In
1931 G. R. Schulze at the University of Minnesota concluded in plain words
what many of his colleagues had phrased more cautiously: ‘Triatomic hydrogen
does not exist. . . . The non-existence of triatomic hydrogen has been definitely
settled’ [39].
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Early history of H3 and H3+
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It should be noted that the H3 hypothesis was not disproved in any strict
sense, but that its credibility was eroded to such an extent that scientists lost
interest in it. Indeed, the hypothesis is an example of what philosophers call an
existential statement: while one can prove or verify such a statement, how can it
possibly be disproved? It is also worth pointing out that the critics aimed their
fire only at the neutral H3 molecule. None of them denied the existence of H+
3 in
discharge tubes.
Finally, in regard to the later development, it is worth noting that, in the
period from 1911 to the late 1930s, there were no suggestions that H3 or H+
3
might be of any interest to the astronomers. The chemical composition of the
higher strata of the atmosphere was at the time a disputed issue, not least in
connection with the origin of the colours of the aurora borealis [40]. While none of
the auroral researchers referred to the possible existence of H3 or H+
3 in the upper
atmosphere, in 1923 a French meteorologist hypothesized that certain phenomena
of atmospheric low pressure might be due to the 3H2 → 2H3 contraction effect
argued by Wendt and others [41]. This seems to have been the only suggestion
that triatomic hydrogen might play a role outside the laboratory.
7. Concluding remarks
The case of the early history of triatomic history is of interest not only for
historical reasons but also because it illustrates certain general features in the
scientific process. First of all, it is a nice example of the evidential nature of
scientific knowledge. Second, it problematizes the notion of discovery. As far as
the question of H3 was concerned, it was all a matter of weighing experimental
evidence for and against the hypothesis of H3 as the source of active hydrogen. For
a period in the early 1920s, the H3 gas was ‘almost discovered’, in the sense that
its existence was considered plausible by a large part of the scientific community.
Yet, evidence claims can always be questioned, or they can be challenged by
counterevidence, and this is what happened. By the late 1930s, H3 had effectively
been de-discovered—only to be re-discovered some 40 years later! That happened
when Herzberg unexpectedly found spectral lines from discharge tubes that he
attributed to Rydberg states of H3 [42]. One may similarly reflect on the discovery
history of H+
3 , a molecular ion that apparently was discovered twice, by Thomson
in 1911 and by Takeshi Oka in 1980 [43,44]. Moreover, the discoveries of Herzberg
and Oka were closely intertwined, as they both occurred in searches for the
H+
3 spectrum at the Herzberg Institute of Astrophysics in Ottawa, Canada.
On the occasion of Oka’s 60th birthday, Herzberg (at the age of 88) referred
to Oka’s first observation of the H+
3 spectrum as the foremost example of his
‘extraordinary persistence . . . and imagination’ [45]. While Herzberg’s discovery
was serendipitous, Oka’s was not.
There is also a historiographical aspect of the case presented here. Together
with other cases of a similar nature, it exemplifies ‘presentism’, which is a natural
element in much writing of history. Historians deal with the past, and what
happened in the past cannot possibly be affected by the present. Nonetheless,
our perspective on the past and the significance of past events can and often do
depend on the later situation. The main reason why the early history of triatomic
hydrogen is of more than antiquarian interest is that H+
3 plays such an important
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5234
H. Kragh
role in modern research [46]. Had this not been the case, the early history would
have appeared much less interesting (and irrelevant to modern scientists) and
possibly not worth investigating in any detail.
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