In the fall of 1987 during a photoacoustic study of hydrogen peroxide's fourth vibrational overtone band, the remarkably clear and well resolved spectrum was observed. Its periodicity and K subband structure suggested that the spectrum was not of hydrogen peroxide but of a smaller near-prolate symmetric top molecule. Furthermore, the time dependent behavior of the signal indicated that chemical changes were taking place within the photoacoustic apparatus

Reilly, James P.

English

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NASA-CR-192136
NASA Research Grant NAGW-1670 Final Report _,/D
James P. Reilly, Dept. of Chemistry, Indiana University
I. Free Radical Spectroscopy
In the fall of 1987 during a photoacoustic study of hydrogen peroxide's
fourth vibrational overtone band we observed the remarkably clear and well
resolved spectrum displayed in Figure IA. Its periodicity and K subband
structure suggested that the spectrum was not of hydrogen peroxide but of a
smaller near-prolate symmetric top molecule. Furthermore, the time dependant
behavior of the signal indicated that chemical changes were taking place within
the photoacoustic apparatus. _ _2
A molecule that contains only hydrogen and oxygen and has roughly
appropriate rotational constants to match the observed spectrum is the OOH
radical. Its ground state rotational constants are as follows:
A"-20.35656 cm "x B"-I.I17901 cm -x C"-I.05643 cm "I
The radical's large A constant leads one to expect large spacings between K
subbands. OOH's relatively small and approximately equal B and C rotational
constants should lead to small spacings within K subbands. Qualitatively, these
expectations matched the observed patterns and we tentatively identified the
molecule as OOH.
Rotational simulations were then conducted using the literature ground
state rotational constants for the OOH radical (Figure IB). Fine tuning of the
upper state rotational constants was accomplished with a simplex least squares
optimization program using the assignments and wavelength information from the
experimental spectrum. While the overall position of the K subbands was in good
agreement with experiment, the line spacings and band head positions within the
subbands did not match exactly. A more detailed, high resolution experimental
spectrum was necessary before the ambiguity within the K subbands could be
resolved.
We recorded high resolution, individual K subband spectra with a Spectra
Physics 380D ring dye laser operating single mode with the photoacoustic cell
positioned within the laser cavity. A complete spectrum was generated by adding
together many smaller overlapping scans each typically 1 cm -I in length.
Examples of these high resolution K subband scans are shown in Figures 2 and 3.
The dropoff in signal intensity observed for the five R branches at long
wavelengths is in part due to chemistry within the photoacoustic cell reducing
the population of the radicals. Fewer rotational lines were observed for higher
subbands since J>_K. After 5-6 hours all traces of the spectrum disappeared and
a hydrogen peroxide spectrum could then be recorded.
Comparison of high resolution experimental spectra and simulations (e.g.,
Figures 4A and 4B) indicated a serious discrepancy. The spacings within and i
between P, Q and R branches were clearly not correct. In order to fine tune the
upper state rotational constants another simplex least squares optimization was
conducted using the more precise high resolution spectral data. Under these
conditions the program could not match the data to within its experimental
accuracy.
Further manipulation of the simulation's rotational constants provided a
clue as to the problem. Much better agreement between experiment and simulation
could be obtained by varying both the upper and lower state rotational constants.
Since this was the case it was no !ongerclear that OOH was responsible for the
(NASA-CR-1921_6) FREE RADICAL ANO
OVERTONE SPECTROSCOPY Final Report
(Indiana Univ.) 10 p
N93-21555
Uncl as
https://ntrs.nasa.gov/search.jsp?R=19930012366 2020-03-17T06:29:27+00:00Z
spectrum that we have been observing.
Our experimental procedure for producing the unknown spectrum was re-
evaluated. In order to produce the spectrum it was necessary to completely
disassemble, clean and reassemble the photoacoustic apparatus. A photoacoustic
cell design that contained o-rings instead of epoxy failed to produce the desired
spectrum unless it was first exposed to the epoxy. Mass spectroscopic analysis
of the epoxy outgassing products indicated the presence of diethylenetriamine,
a primary/secondary amine known to react with peroxides. By introducing pure
diethylenetriamine into o-ringed photoacoustic cells that had yielded no data,
beautiful spectra were immediately generated.
It was now clear that more elements than just hydrogen and oxygen were
present in the photoacoustic apparatus and that the range of possible species
capable of producing the rotationally resolved spectrum would have to be
expanded. The OOH radical did have a geometry close to what was necessary but
increases to both the B and C rotational parameters were found to improve the
experimental/slmulation agreement. An increase in the B and C rotational
constants results from replacing one of the oxygen atoms in OOH with an atom of
lower mass. This has the effect of reducing the moment of inertia about the B
and C rotational axis and increasing the rotational constants. The A rotational
constant is largely unaffected by this change as the mass change is virtually
along the A rotational axis.
Of the possible hydrogen, nitrogen, carbon and oxygen arrangements, the
molecule HNO became a prime suspect. Rotational constants for HNO have been
previously obtained by flash photolysis, microwave spectroscopy, and
microwave/vlslble double resonance methods. These rotational constants are as
follows:
A'-21.95460 cm "I
A"-I8.A6343 cm -z
B'-1.325500 cm -I
B"-l.41028A cm "l
C'-1.242600 cm -I
C"-1.305649 cm "z.
These values were then used to perform another rotational simulation (fig. 4C).
The simulation's line spacings were found to be in good agreement with experiment
for subbands near the origin. (Other simulated subbands were also in good
agreement but they suffer from K-dependant rotational perturbations.)
KNO is known to have an electronic AIA"_XIA ' transition occurring at 760.2
nm. This wavelength compares well with our estimated transition origin of 760
nm. We therefore conclude that we have been recording A'A"_X'A' (0,0) spectra
of HNO produced by the reaction of HzO z with diethylenetriamlne, a component of
torr-seal epoxy.
Superb spectra of this transition have already been recorded by Dalby [Can.
J. Phys. 36, 1336 (1958)] and Ramsay [Can. J. Phys. A0, 322 (1962)] and we
decided that our measurements were not likely to improve the current state of
understanding of }{NO so this project was terminated.
II. Overtone Spectroscopy
In the last several months we have returned to the vibrational overtone
field and have been recording photoacoustic spectra with a new titanium sapphire
laser. The 3v I + w_ + w 5 band of the methyl acetylene is displayed in Figure 5
along with an asymmetric top rotational simulation. Each peak in this spectrum
is really an unresolved subband. It is easy to see both the resemblance and the
difference between Figures 5A and 5B. The subbands on either wing of this band
match the simulation very well while those near the central Q branch appear
highly disorganized. Several more subbands are experimentally observed than are
jf
predicted and their intensities are quite irregular. Nevertheless, we were able
to assign every peak in Figure 5A. This was accomplished by writing a simple
computer program that correlates the lines in the P branch with those in the R
branch. By taking advantage of the fact that splittings of rotational energy
levels will be manifest in both branches, we were able to construct the energy
level diagram displayed in Figure 6 and we assigned quantum numbers to every
peak in the observed spectrum. We would like to identify the vibrational state
couplings and the exact nature of the vibrational levels that mix in order to
produce the observed splittings. While the clear evidence of a J dependence to
the splittlngs might initially suggest a Coriolis coupling mechanism, two facts
argue agalnst this. First, the J-O level of the excited state is split. Second,
the P and R branches both simplify as the angular momentum quantum number
increases. This is opposite of what is expected for Corlolis coupling. Next
we considered an anharmonlc coupling model. Since the splittings evident in
Figure 10A are relative large, (up to about 1 cm -I) we believe that if anharmonlc
coupling is their source it would probably be a rather efficient (e.g. third or
fourth order) process. With a harmonic oscillator basis set, we worked out all
possible vibrational levels that could anharmonlcally couple with 3w I + w 3 +w 5
and that occur within I00 cm "z of its 12764 cm "I position. We found that there
were 19 such levels, so at this time we cannot identify which two or three are
causing the dramatic effects evident in Figures 5 and 6.
A second intermediate-size molecule that we have been studying very
recently is methane. Although the vibrational bands of this spherical top
molecule might be expected to exhibit rather simple rotational structure, its
Av-3 C-H overtone spectrum has only been partially analyzed; all higher overtones
are uninterpretable. The source of the observed complexity is not difficult to
identify: of the four normal modes of vibration, one is doubly degenerate and
two are triply degenerate. As a result, even the fundamental w 3 and u 4 infrared
bands exhibit complex tetrahedral fine structure due to rotation-vlbration
interactions. Following considerable theoretical and experimental effort this
fine structure is now well understood in the fundamental bands. In fact the
ground and first vibrational levels are exceedingly well characterized. However
the degeneracies of excited states accessed by overtone transitions are higher.
To make matters worse, two of the normal modes are near 1500cm "I and two are near
3000cm -1. The opportunities for anharmonlc and Corlolis interactions to split
degeneracies and couple various hlgh-lying vibrational states are proliferous.
We have recorded the 3w I + w 3 overtone spectrum of methane under a few
different conditions. The room temperature low resolution (approximately 0.2
cm -l) spectrum is displayed in Figure 7A. In order to reduce the spectral
congestion we also recorded this spectrum at 90 K using our cryogenic
photoacoustlc cell. The result is displayed in Figure 7B. Finally, a simple
spherical top simulation, calculated at 90 K, appears in Figure 7C. At the
present time we can assign some but not all of the peaks in the experimental
spectrum. In the near future we will be extending this work through single mode
photoacoustic and molecular beam spectroscopic experiments. A research proposal
on this topic has just been submitted to the NASA Planetary A_mospheres division.
Two or three manuscripts will be submitted for publication based on the
work described above.
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(K subbands are labelled)
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Figure 3: More high resolution experimental daca
Figure 4
4-3 PC_RBRANCHES
EXPERIUENTAL
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FigureS: 3_I*V3÷_5vibrationalvertonebandinthe
near-lnfrared. (A) experimental spectrum
(B) symmetric top simulation.
unper- tur'bed
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Figure 6 : Rotational energy level diagram for the methyl
acetylene 3_ 1 + _3 + _5 excited vibrational
state. On the left are unpeturbed levels
predicted by a symmetric top simulation. On
the right are the experimentally determined
perturbed energy levels, based on an analysis
of the spectrum in Figure 10A.
111 07 cm-1 11 385 cm-1
B !i
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Fi__ure 7 : Bulk gas Av-4 over=one s_ec=ra of me=bane
(A) 295 K (B) 90 K
(C) 90 K slmula=ion (nuclear spin sea=is=its and :o=rahedral fine
s_ruc=ura neg!ec=ed)


Free radical and overtone spectroscopy

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