David Dayton

The structure of the carbon dioxide dimer from near infrared spectroscopy

An F‐center laser–molecular beam spectrometer has been used to obtain a sub‐Doppler resolution infrared spectrum of the carbon dioxide dimer. The vibrational mode investigated in this study corresponds to the ν1+ν3 combination mode of the monomer located at 3716 cm−1. A qualitative assignment of the spectrum shows unambiguously that the equilibrium structure of the dimer is the slipped parallel, rather than the T‐shaped, geometry. The observed spectrum cannot be fit to within experimental error using conventional asymmetric rotor formalism. This may be due to a number of factors such as Fermi resonance between the upper state levels of the band and nearby levels of the dimer, such as seen in the monomer, or it could arise from tunneling effects arising from the two large amplitude motions in the dimer.

Photofragment angular distributions for HF dimer: Scalar J–J correlations in state‐to‐state photodissociation

Photofragment angular distributions have been measured for HF dimer which show resolved structure that can be assigned to individual fragment rotational channels. This data is used to establish intermolecular scalar correlations between the rotational states of the two HF fragments. The observed angular distributions are strongly dependent upon whether the ‘‘free’’ or ‘‘hydrogen bonded’’ HF stretch is initially excited. Since the infrared spectrum of the parent molecule is highly resolved, these results can be used to determine the relative state‐to‐state photodissociation cross sections. In addition, the zero point dissociation energy (D0 ) of the HF dimer is accurately determined.

Mode-dependent vibrational predissociation in the HCN-HF binary complex

Abstract Sub-Doppler resolution near-infrared spectra have been obtained for the ν 1 and ν 2 bands of HCN-HF. The molecular constants obtained from these spectra are in excellent agreement with those obtained in previous studies. The high resolution of the present experiments has enabled us to obtain accurate values for the vibrational predissociation lifetimes, namely 0.58(1) × 10 −10 s and 1.35(5) × 10 −8 s, for the ν 1 and ν 2 bands, respectively. This dramatic difference in lifetimes clearly shows that the vibrational predissociation dynamics is highly mode dependent in this system.

Infrared and microwave spectra of OCO–HF and SCO–HF

The H–F stretching bands of the OCO–HF and SCO–HF complexes have been studied by optothermal (bolometer‐detected) molecular‐beam spectroscopy. Both species exhibit spectra of a quasilinear molecule red shifted from free HF by 52.1 and 57.5 cm−1, respectively. The principal band in both molecules is accompanied by a slightly red‐shifted doublet‐type subsidiary band that can be interpreted as a hot band of a low frequency bending vibration or a K=1 subband of a bent molecule. Accurate doublet splittings in the ground H–F vibrational state have been measured by pulsed‐nozzle Fourier‐transform microwave spectroscopy.

Infrared spectroscopy of the bent isomer of N2O-HF

Abstract We report the infrared spectrum of the bent isomer of N2O-HF in which the HF subnit is hydrogen bonded to the oxygen. This isomer was previously observed using microwave spectroscopy, while recent infrared measurements by Lovejoy and Nesbitt have shown that a linear (or slightly bent) ONNHF isomer also exists. We find that the linear ONNHF isomer is present under identical beam conditions to those used to record the spectrum of the bent isomer.

Infrared spectroscopy and ab initio theory of the structural isomers of CO2–HCN

Infrared spectra have been obtained for the liner and T‐shaped isomers of CO2–HCN using the optothermal detection method. These spectra correspond to the intramolecular C–H stretching vibration in each isomer. The infrared results are consistent with the structures determined previously from microwave spectroscopy and provide additional data that is helpful in making meaningful comparisons with theory. Ab initio calculations have also been carried out for this system using three different basis sets. The agreement between the experimental and calculated structures is good for all three basis sets, while the vibrational frequency shifts (scaled to the monomer) and the intermolecular stretching force constants are much more basis set dependent. Nevertheless, the overall agreement between experiment and ab initio theory is very good.

The lowest-frequency bending mode (ν71) of HCN-HF from near-infrared laser spectroscopy

Abstract The lowest intermolecular bending frequency of HCN-HF has been determined in both the ground (73.583(3) cm −1 ) and excited (73.176(3) cm −1 ) CH vibrational states using near-infrared laser molecular beam spectroscopy. The results also show that the vibrational predissociation lifetime is weakly dependent on the intermolecular bending state.

Infrared spectroscopy of the HCN-(HF)2 ternary complex

Abstract The opto-thermal detection method has been used to obtain the first gas-phase infrared spectrum of the non-linear HCN-(HF) 2 complex. Rotational constants in the ground and vibrationally excited states have been determined, as well as the band origin and vibrational predissociation lifetime. The experimental spectrum is a direct result of the excitation of the C-H stretching vibration in the complex. Two other featureless vibrational bands have been observed which are tentatively assigned to the HF stretches in this complex.

The fundamental C–H stretching vibration and associated intermolecular bending hot band of SCO–HCN

Infrared optothermal laser spectroscopy has been used to study the linear SCO–HCN complex. The observed spectrum corresponds to excitation of the C–H stretching vibration out of either the ground state or v=1 of the lowest frequency intermolecular bending state. Ab initio calculations have also been carried out for this system which give, at least for some properties, results which are in good agreement with experiment. A comparison between the SCO–HCN results and those of the linear CO2–HCN complex studied previously, reveals that the shifts in the C–H vibrational frequency upon complex formation are rather similar. This is despite the fact that OCS and CO2 have very different point multipoles, suggesting that the electrostatic part of the interactions are likewise different for these two partners. Nevertheless, a distributed multipole calculation shows that, at the intermolecular separations characteristic of these complexes, the electrostatic interactions are in fact quite similar at the oxygen ends of...

Gas-phase infrared-spectroscopy of cyclopropane-HF and cyclopropane-HCN

Abstract The optothermal detection method has been used to obtain gas-phase infrared spectra of the cyclopropane-HF and cyclopropane-HCN binary complexes, both of which show substantial homogeneous broadening. Shifted somewhat from the spectrum of the HF binary complex is an additional band which is tentatively assigned to a larger complex which contains both cyclopropane and HF.