Kragh: Discovery of H3 and H3+
The childhood
+
of H3 and H3
Helge Kragh tells the story of the discovery of triatomic hydrogen.
Initially a controversial species whose existence was in doubt, it has
become a cornerstone of astrochemistry.
T
he H3+ ion is one of the most abundant
ions in the universe and has been detected
both in interstellar clouds and the ionospheres of Jupiter, Saturn and Uranus. The interdisciplinary science concerned with H3+ is today
cultivated by hundreds of physicists, astronomers
and chemists (Geballe and Oka 2006). While triatomic hydrogen H3 and its cation H3+ have been
studied since 1911, when their existence was
inferred from experiments with positive rays,
the existence of H3 remained controversial, and
by 1930 belief in the molecule was in decline.
In spite of the considerable amount of work
devoted to triatomic hydrogen in the interwar
period, none of it included an astronomical
dimension. Molecular astrochemistry emerged
only in the late 1930s when the Belgian astronomer Pol Swings and his compatriot, the physicist
Léon Rosenfeld (a close collaborator of Niels
Bohr), argued that an unidentified absorption
line arose from CH molecules in the interstellar
medium. Four years later a few more molecules
and the first molecular ion had been added to
the list: NH, CN and CH+. This was the modest
beginning of a research field that only began
to flourish in the 1960s and since then has
expanded into a major interdisciplinary science
(Herzberg 1988, Fraser et al. 2002).
The Herzberg connection
In 1961, in what is possibly the first reference to
the astronomical significance of H3+, three American physicists suggested that the ion would probably be abundant in interstellar space, formed by
the reaction H 2 + H 2+ → H3+ + H. “It now appears
desirable,” they wrote, “to consider the possibilities for detecting H3+ because this molecular ion
may be present under some circumstances to the
virtual exclusion of H 2+,” (Martin et al. 1961).
To “detect” a molecule means in this context
to find its spectrum or parts of a spectrum, and
this was only achieved by Takeshi Oka in 1980,
the culmination of a research project that had
started five years earlier. Working at what since
1975 had been the Herzberg Institute of Astrophysics, Ottawa, Oka succeeded in detecting the
first 15 spectral lines of the infrared absorption
A&G • December 2010 • Vol. 51 1: Stark’s model of triatomic hydrogen
as depicted in his book Prinzipien der
Atomdynamik from 1915.
spectrum of H3+ (Oka 1980, 1983). His pioneering identification was experimental, and it was
another 16 years before the spectrum was found
in interstellar space.
Claims that the neutral H3 molecule had been
positively identified had been made both before
and after the second world war. However,
because spectral evidence was missing these
claims were not generally accepted. The situation
changed in 1979, shortly before Oka’s discovery
of the H3+ spectrum. Searching for the infrared
emission spectrum of H3+, the Nobel laureate
and pioneer of molecular spectroscopy Gerhard
Herzberg unexpectedly found lines from cathode
discharge tubes that he attributed to the Rydberg
spectrum of the unstable molecules H3 and D3
(Herzberg 1979, Stoicheff 2002). Herzberg’s discovery – made at the age of 75 – was recognized
as a breakthrough in molecular science. In 1994,
on the occasion of his 90th birthday, he was
asked in an interview for the most satisfying of
his discoveries. Herzberg answered: “H3 would
definitely be one of my favourites because it was
quite unexpected. We were actually looking for
positively charged H3+ at the time but found neutral H3 instead,” (Reichle 2002).
Triatomic hydrogen was far from a new molecule to Herzberg. More than 50 years earlier,
while finishing his doctoral dissertation at the
Darmstadt Technische Hochschule, he had an
interest in the H3+ ion and the subject was part
of his dissertation of 1927. Heinrich Rau, Herzberg’s professor in Darmstadt, was a specialist
in positive rays. In an interview of 1983, Herzberg recalled: “Of course, there was no clear
distinction between H3+ and H3 at that time …
Rau thought that, now, if we look at the spectrum of a discharge, we find the atomic lines
and we find these many lines, so-called many
line spectrum. Is this many line spectrum due to
H 2 or H 2+ or H3+ maybe? … I was quite fascinated
by that,” (AIP 1983).
Early models of H3
During the years 1906 to 1914, J J Thomson
conducted an extended series of experiments
with positive rays. In some of these he reported a
secondary radiation made up of molecular ions
with a m/e ratio three times that of the ordinary
hydrogen ion H+. He argued that the molecule,
which he mostly referred to as X3, was likely
to be triatomic hydrogen, a hypothesis he first
stated in 1911. Over the next three years Thomson provided further evidence for the X3 = H3
hypothesis, which he described in the Bakerian
Lecture of 1913 and in a monograph of the
same year, Rays of Positive Electricity. What
Thomson had found was the H3+ ion, but he was
convinced that it was merely the charged version
of a stable molecule that was nearly chemically
inert. In spite of all conflicting evidence he stuck
to his belief in a stable triatomic form of hydrogen. As late as 1937 the 81-year-old physicist
concluded that “though H3+ is so evanescent, H3
itself is much more durable” (Thomson 1937).
Triatomic hydrogen was a chemical enigma.
Thomson was well aware that the existence of
H3 conflicted with ordinary ideas of valency, but
he suspected that these ideas would have to be
modified in the light of the modern theories of
atomic structure. In 1913 he indicated a possible
arrangement and the same year another prominent physicist, Johannes Stark in Germany, suggested a more detailed picture of the molecule
(figure 1). Stark believed that H3 agreed with his
own unorthodox ideas of valence and molecular
constitution, indeed that it provided “strong support” for them. According to him, a triatomic
ring of hydrogen atoms could be formed by three
atoms and three electrons held in an equilibrium
position by electric lines of force (Stark 1913).
However, Stark’s pictorial model of H3 failed to
attract interest, which was also the case with an
even more speculative model proposed in 1915
by the American physicist Albert Crehore.
From the perspective of the period there was
no obvious reason why the H3 molecule should
not exist and be stable. In his pathbreaking
atomic theory of 1913, Niels Bohr had offered a
quantitative model of H 2 and, in 1919 in a little
known paper published by the Nobel Institute
in Stockholm, he extended his calculations to
6.25
Kragh: Discovery of H3 and H3+
2: Bohr’s 1919 linear models of H3 (left) and H3– (right). In the neutral
molecule, three electrons rotate in a common circular orbit of radius
a, and the three nuclei (protons) are placed on the axis of the orbit,
separated by the distance b. In the ion, the four electrons rotate in two
circular orbits between the two outer nuclei. For the ground state of the
two systems Bohr calculated a = 0.99 a0 (H3) and a = 1.08 a0 (H3–), where a0 is
the Bohr radius of the hydrogen atom.
the H3 system (Bohr 1982). His model (figure 2)
consisted of three electrons rotating in a common circular orbit, with one nucleus at the centre and the other two symmetrically displaced
from it:
“The model of a hydrogen molecule
containing three atoms, which, although in
chemical sense it would not show the same
degree of stability as the ordinary diatomic
molecule, should still possess the possibility of a permanent existence if undisturbed
by external agencies. This model may
therefore possibly correspond to the molecule of a new modification of hydrogen,
for the appearance of which under suitable
conditions interesting evidence has been
brought forward by Sir J J Thomson in his
well known experiments on positive rays.”
Bohr found that the process 3H 2 → 2H3 was
endothermic but that it would occur in a hydrogen gas ionized by, for example, the action of
alpha rays. Apart from neutral H 2 molecules,
such a gas would contain H+ and H 2–, leading to
triatomic hydrogen by the exothermic process
H+ + H 2– → H3. He further found that it would
only take a small external action to break up
the H3 molecule into H and H 2 .
Bohr’s investigation focused on H3 rather than
H3+, his reason being that for the latter system
“no configuration of mechanical equilibrium,
in which the nuclei are at rest at finite distances
from each other and the electrons move in
circular orbits, can be formed.” If an electron
were removed from H3, the ion would split up
as H3+ → H 2 + H+. On the other hand, Bohr’s
calculations showed that H3 could add another
electron and form a H3– ion. This anion, he predicted, “may exist permanently in the absence of
external agencies.” Although Bohr’s model calculations made sense within the framework of
the old quantum theory, they concerned a hypothetical molecule and exerted very little influence
on the further discussion of triatomic hydrogen
and molecular constitution in general.
The models of Thomson, Stark, Crehore and
Bohr were not the only ones of H3 before the
emergence of quantum mechanics. Herbert
Stanley Allen, professor of physics in the University of St Andrews, thought that the hydrogen molecules H 2 and H3 were important test
cases for theories of valency. However, in agreement with the view of most chemists, he found
Bohr’s dynamical model to be unsatisfactory.
6.26
Allen consequently suggested replacing it with a
model where the covalent bond was represented
by a single fixed electron, a view he justified by
the ad hoc introduction of a “quantum force”
varying inversely as the cube of the distance.
In 1923 he described his alternative as follows:
“A possible configuration may be suggested for
a neutral triatomic hydrogen molecule, H3, in
which the nuclei and electrons are situated at
alternate corners of a regular hexagon (length of
side, 0.625 Å),” (Allen 1923). For the complete
dissociation energy into three hydrogen atoms
he obtained 46 eV. In qualitative terms Allen’s
model resembled the earlier one of Stark, but like
Bohr’s model it was developed mathematically.
Active hydrogen
The early history of H3+ differed markedly from
that of H3. Thomson’s discovery of H3+ in discharge tubes was confirmed by other researchers,
first by the young Canadian–American physicist
Arthur Dempster in experiments of 1916 (figure
3). However, Dempster found that H3 did not
exist as a stable gas, contrary to what Thomson believed (Dempster 1916). Later experiments made by British, American and German
physicists gave results in broad agreement with
those obtained by Dempster. For example, the
Princeton physicist Gaylord Harnwell found in
positive-ray experiments that most of the H3+ was
produced in the reaction H 2 + H 2+ → H3+ + H, such
as first suggested by T R Hogness and E G Lunn
at the University of California two years earlier
(Harnwell 1927). Other experiments, done with
improved versions of the mass spectrograph, left
no doubt about the existence of H3+ (Bainbridge
1933). Yet, apart from its existence, little was
known about the ion.
While the existence of H3+ was convincingly
demonstrated by experiments, it was another
matter with the neutral molecule. The possible
existence of an “active” form of hydrogen was a
hot topic in the 1920s, when it was investigated
by dozens of chemists and physicists. It was
often assumed that the new or “ozonic” form
of hydrogen consisted of the H3 molecules that
indirectly were demonstrated in experiments
with positive rays. In a work of 1917, William
Duane and Gerald Wendt examined the action
of alpha rays on pure hydrogen, noticing several
physical and chemical effects that they ascribed
to the formation of H3 (Duane and Wendt 1917).
For example, they found a volume contraction
supposed to be caused by 3H 2 → 2H3. Over the
next decade numerous experiments were made
on active hydrogen, primarily to establish its
properties and secondarily to establish whether
or not it could be explained in terms of H3.
Apart from the contraction effect, the main
characteristics of active hydrogen were:
● it readily attacked mercury, forming a mercury hydride;
● its boiling point was much higher than ordinary H 2;
● it was unstable, with a lifetime of the order
of a minute;
● it combined with nitrogen to form ammonia;
● it formed H 2 S when passing over sulphur
powder.
The latter reaction, easily identified by the
effect on lead acetate on strips of paper, was
often considered a standard test of active hydrogen. The properties were quite different to those
known from ordinary hydrogen and, according to Wendt and several other chemists, they
provided solid evidence for a gas made up of tri­
atomic hydrogen. Wendt and his student Robert
Landauer thought that “the properties of the
new gas are precisely those to be expected of an
ozone form”, and they consequently proposed to
name it “hyzone” (Wendt and Landauer 1920).
Although the name was occasionally used in
the 1920s and 1930s, “hyzone” soon vanished
into oblivion.
For a brief period of time in the early 1920s
evidence seemed to favour an active form of
hydrogen made up of unstable hydrogen molecules. No spectrum of H3 had been produced,
but a few advocates of active hydrogen argued
that there was indirect spectral evidence for the
gas. Thus, Wendt and Landauer interpreted
changes in the hydrogen spectrum at low temperatures as probably due to a gradual formation of H3. It was generally admitted that the
evidence from spectra was at best circumstantial
and that chemical evidence was more convincing. However, from about 1925 the evidence
claims based on chemical effects were increasingly questioned and counterevidence was
produced. The Austrian chemist Fritz Paneth
concluded that the effects ascribed to H3 could
in almost all cases be explained by the presence
of small amounts of H 2S in the hydrogen gas.
Noting that there were “many contradictions in
the past work on active hydrogen”, Harold Urey
and his student Hugh Smallwood were unable to
A&G • December 2010 • Vol. 51
Kragh: Discovery of H3 and H3+
3: Dempster’s spectrum from 1916 of positive
hydrogen rays deflected in a magnetic field.
The symbols H1, H2 and H3 refer to the singly
charged positive ions.
confirm the results upon which the H3 hypothesis rested (Smallwood and Urey 1928). Several
other chemists joined the campaign against
H3, with the result that by the early 1930s the
hypothesis had effectively lost its credibility.
Although the hypothesis was not disproved in
any strict sense, to the mind of most chemists
and physicists the accumulated evidence against
it counted as a de facto disconfirmation.
With the emergence of quantum chemistry in
the late 1920s it became possible to calculate
simple molecules such as H3 and H3+. This was
first done in 1931 by Harrie Massey and, independently, Charles Coulson, who both considered H3+ important because it was the simplest
possible case of a triatomic molecule. In a series
of papers between 1936 and 1938, Joseph Hirschfelder and collaborators made careful calculations of the energy and structure of the
two systems. Hirschfelder argued that some of
the vibration frequencies of H3+ would be in the
infrared region and “therefore susceptible to
direct experimental observation” (Hirschfelder
1938). However, there seems to have been no
search for the lines at the time. With respect to
the question of the existence of H3, the calculations based on quantum mechanics merely confirmed the instability of the molecule. They did
not result in conclusive answers or predictions
that could be tested in the laboratory.
Molecule or isotope?
The confusion that characterized the early history of H3 did not arise solely from contradictory
experiments, but also because of contemporary
ideas about another form of hydrogen of mass
A&G • December 2010 • Vol. 51 3. The notion of isotopy was introduced by
Frederick Soddy and others in 1913, at about
the same time as Thomson proposed his H 3.
As early as September 1913, during a discussion of H3 at the annual meeting of the British
Association for the Advancement of Science,
Bohr suggested that Thomson’s X3 might be a
superheavy hydrogen isotope of mass 3 rather
than molecular H3 (Eve 1939). Such an isotope,
which according to the new Rutherford–Bohr
picture consisted of three protons and two
nuclear electrons, was purely hypothetical at
the time. It was occasionally discussed as an
alternative to the H3 hypothesis.
The Chicago physical chemist William Harkins
speculated in 1920 that “eka-hydrogen”, or what
later would be called tritium, was a constituent
of atomic nuclei (Harkins 1920). Although the
idea of identifying H3 with a hydrogen mass-3
isotope was never seriously entertained, for a
period the possible existence of two forms of
H3 led to some confusion. According to an English chemist, it had been “definitely shown that
H3 carries one charge, and this fact, considered
along with its formation from hydrogen, shows
that it is an isotope of hydrogen” (Baly 1921). As
late as 1934, two years after Urey and coworkers had discovered deuterium, Thomson argued
that what he had found more than two decades
ago was really the heavy isotope of hydrogen in
the form HD and HD+.
Although H 3 and H 3+ did not occur in an
astronomical context in the pre-second world
war era, the hypothetical mass-3 hydrogen isotope did. French physicists studying the radiation emitted from the Orion nebula concluded
that some of the lines must be attributed to an
unknown gas (“nebulium”) of atomic weight
about 3 (Bourget et al. 1914). According to
Harkins, nebulium was likely to be the same
as eka-hydrogen, while the British astrophysicist John Nicholson claimed that the discovery
supported his theory of primordial elements
in nebulae and Wolf–Rayet stars (Nicholson
1914). It took another 13 years until the riddle
of nebulium was finally solved, namely when Ira
Bowen identified the lines as transitions from
metastable states of doubly ionized oxygen and
nitrogen atoms.
Before 1930 there was no suggestion of H 3
or other molecules in space, but there was at
least one proposal that H 3 might play a role
in the higher strata of the Earth’s atmosphere.
Assuming that hydrogen was abundant at altitudes higher than 80 km, a French meteorologist by the name Joseph Lévine thought that
the triatomic hydrogen of Wendt and Landauer
was present too (Lévine 1923). He proposed
that certain phenomena of atmospheric low
pressure might be explained by the contraction
effect observed by Wendt and others. Lévine’s
speculation was ignored by other meteorologists
and atmospheric scientists.
Conclusions
Like other fields of history, the history of science depends not only on the past but also on
the present. Events in the past may change their
meaning and significance as history unfolds.
A scientist or historian writing the history of
triatomic hydrogen in 1940 would probably
do it briefly, describing the case as a scientific
mistake that was eventually corrected. Thomson’s H3+ would be left as the single and isolated
achievement. Because of the much later recognition of the reality of H3, and especially of the
great astronomical importance of H3+, from the
vantage point of today the early history appears
in a very different light.
The case of H3 illustrates the evidential nature
of scientific knowledge. By 1920 chemists and
physicists were faced with experiments that indicated the existence of an unusual form of hydrogen. Was active hydrogen a reality? If so, did it
consist of triatomic molecules? The questions
could be answered neither by theory nor by a
crucial experiment. The only way was by weighing evidence for and against the hypothesis. The
result was not a disproof of the hypothesis, but
a consensus view that there was no good reason
to assume that H3 existed elsewhere than in the
mind of the theorists. We know today that H3
does exist, which only illustrates the temporary
nature of consensual knowledge. ●
Helge Kragh, Dept of Science Studies, Aarhus
University, Denmark.
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