Gerald T. Fraser

van der Waals potentials from the infrared spectra of rare gas–HF complexes

High‐resolution infrared spectra of the Ar–HF, Kr–HF, and Xe–HF van der Waals molecules have been recorded in the vicinity of the H–F stretching fundamentals, ν1, under thermal equilibrium conditions at T≂211 K with a tunable difference‐frequency laser. Rotational structure has been observed up to or approaching rotational predissociation, permitting us to model the effective radial van der Waals potentials for these complexes. These potentials provide good estimates for the binding energies, D0, and the van der Waals stretching frequencies, ν3, in the ground (v1=0) and excited (v1=1) states of the molecules. For v1=0 in Ar–HF, Kr–HF, and Xe–HF, we find D0=102, 133, and 181 cm−1 and ν3=39.2, 41.1, and 43.4 cm−1, respectively. The ν3 modes characterized by the model potentials aid in the assignment of the ν1+ν3−ν3 hot bands observed in our spectra. The band centers for the ν1 fundamentals are all down shifted in frequency from the isolated HF monomer by Δν=−9.654, −17.518, and −29.185 cm−1 for the Ar, Kr, ...

Structure and vibrational dynamics of the CO2 dimer from the sub‐Doppler infrared spectrum of the 2.7 μm Fermi diad

Sub‐Doppler infrared spectra of two Fermi resonance coupled bands of carbon dioxide dimer have been obtained at 3611.5 and 3713.9 cm−1 using an optothermal molecular beam color‐center laser spectrometer. The band origins for the complexes are red shifted by approximately 1 cm−1 from the corresponding ν1+ν3/2ν02+ν3 CO2 bands. The higher frequency band is perturbed while the lower frequency band appears free of extraneous perturbations as determined from a precision fit to a Watson asymmetric rotor Hamiltonian. This fit and the observed nuclear spin statistical weights reveal that the complex is planar with C2h symmetry. The C‐‐C separation and C‐‐C–O angle are determined to be 3.599(7) A and 58.2(8)°, respectively. The nearest neighbor O‐‐C distance is 3.14 A which is the same as that found in the crystal. From the centrifugal distortion analysis the weak bond stretching and symmetric bending frequencies are estimated to be 32(2) and 90(1) cm−1. No interconversion tunneling is observed.

Does Ammonia Hydrogen Bond

Spectroscopic characterizations of the stereochemistry of complexes of ammonia (NH3) have strongly confirmed some long-held ideas about the weak interactions of NH3 while casting doubt on others. As expected, NH3 is observed to be a nearly universal proton acceptor, accepting hydrogen bonds from even some of the weakest proton donors. Surprisingly, no evidence has been found to support the view that NH3 acts as a proton donor through hydrogen bonding. A critical evaluation of the work that has been done to gather such evidence, as well as of earlier work involving condensed-phase observations, suggests that NH3 might well be best described as a powerful hydrogen-bond acceptor with little propensity to donate hydrogen bonds.

SUB-DOPPLER INFRARED SPECTRUM OF THE CARBON DIOXIDE TRIMER

A spectrum of the carbon dioxide trimer van der Waals species has been recorded near 3614 cm−1 at sub‐Doppler resolution using an optothermal (bolometer‐detected) molecular‐beam color‐center laser spectrometer. A planar, cyclic structure with C3h symmetry has been determined for the complex with a carbon–carbon separation of 4.0382(3) A. The observed perpendicular band, corresponding to an in‐plane E′‐symmetry vibration of the trimer, has been attributed to a localized excitation of the 2ν02 +ν3 combination mode of a CO2 subunit by virtue of its small blue shift (∼0.98 cm−1) from that of the isolated monomer.

Infrared and microwave investigations of interconversion tunneling in the acetylene dimer

A sub‐Doppler infrared spectrum of (HCCH)2 has been obtained in the region of the acetylene C–H stretching fundamental using an optothermal molecular‐beam color‐center laser spectrometer. Microwave spectra were obtained for the ground vibrational state using a pulsed‐nozzle Fourier transform microwave spectrometer. In the infrared spectrum, both a parallel and perpendicular band are observed with the parallel band being previously assigned to a T‐shaped C2v complex by Prichard, Nandi, and Muenter and the perpendicular band to a C2h complex by Bryant, Eggers, and Watts. The parallel band exhibits three Ka=0 and three asymmetry‐doubled Ka=1 series. The transitions show a clear intensity alternation with Kc with two of the Ka=0 series missing every other line. In addition, the perpendicular band has the same ground‐state combination differences as the parallel band. To explain these apparent anomalies in the spectrum, we invoke a model consisting of a T‐shaped complex with interconversion tunneling between f...

Optothermal‐infrared and pulsed‐nozzle Fourier‐transform microwave spectroscopy of rare gas–CO2 complexes

Sub‐Doppler infrared spectra of Ne–CO2, Ar–CO2, and Kr–CO2 have been recorded near 3613 and 3715 cm−1, in the region of the 2ν02+ν3/ν1+ν3 Fermi diad of CO2, using an optothermal molecular‐beam color‐center laser spectrometer. In addition, pulsed‐nozzle Fourier‐transform microwave spectra are reported for the ground vibrational states of the complexes. The infrared and microwave spectra are consistent with T‐shaped complexes as shown originally by Steed, Dixon, and Klemperer for Ar–CO2.1 The infrared band origins for the Ar and Kr complexes are red shifted, from that of free CO2, by 1.09 and 0.95 cm−1 for Ar–CO2 and by 1.97 and 1.76 cm−1 for 84Kr–CO2. For Ne–CO2, blue shifts of 0.15 and 0.19 cm−1 are observed. The lower Fermi components are free of perturbations, whereas the upper components of Ar–CO2 and Kr–CO2 are perturbed. For Ar–CO2 the perturbation is strong, shifting the positions of the observed Q‐branch lines of the Ka =1←0 subband by as much as 500 MHz.

Ammonia dimer: Further structural studies

New experimental results on the structural and dynamical properties of NH3 dimer are reported in this work. J=1–0, K=0 transitions of 14NH3–15NH3, 15NH3–14NH3, ND3 dimer, and ND3–ND2H have been measured at high resolution and 14N electric quadrupole coupling constants are reported for each of these species. The NH3 subunits comprising the dimer are inequivalent. The quadrupole coupling constant associated with the first ammonia subunit eqQ1aa, is measured in 14NH3–15NH3 [−627(8)kHz], in ND3 dimer [−531(15) kHz], and in ND3–ND2H [−991(18) kHz]. For the other subunit, eqQ2aa is reported in 15NH3–14NH3 [892(8)kHz], in ND3 dimer [745(13) kHz], and in NH3–ND2H [1013(18) kHz]. These numbers can be used to estimate the vibrationally averaged polar angles of these isotopomers of NH3 dimer. The result is (including the primary isotopomer) θ1 for 14NH3–14NH3 is 48.6°, for 14NH3–15NH3 is 48.7°, for ND3 dimer is 49.6° and for ND3–ND2H is 45.3°; while θ2 for 14NH3–14NH3 is 64.5°, for 15NH3–14NH3 is 64.3°, for ND3 dime...

Pulsed‐nozzle Fourier‐transform microwave spectroscopy of laser‐vaporized metal oxides: Rotational spectra and electric dipole moments of YO, LaO, ZrO, and HfO

The rotational spectra of YO, LaO, ZrO, and HfO have been measured using a Fourier‐transform microwave spectrometer in combination with a laser‐ablation source. Here, a Q‐switched Nd:YAG laser (532 nm) was used to vaporize the metal oxides from a target source rod located in the throat of a pulsed‐molecular‐beam valve. A description of the instrument is given. The electric dipole moments of the four species have been measured and compared to ab initio results, where available. The experimental values are μYO =4.524(7), μLaO =3.207(11), μZrO =2.551(11), and μHfO =3.431(5) D. Of special note are the extremely large nuclear quadrupole coupling constants, eQq, determined for the 177HfO and 179HfO isotopic species, with values of −5952.649(35) MHz and −6726.981(39) MHz, respectively. This is the first determination of nuclear quadrupole coupling constants for a molecule containing the Hf atom.

Microwave electric‐resonance optothermal spectroscopy of (H2O)2

The microwave spectrum of (H2O)2 has been measured between 14 and 110 GHz using a newly developed electric‐resonance optothermal spectrometer (EROS) described here. The reported measurements extend previous results on the a‐type Ka=0–0 and 1–1 bands for the A±2 , B±2 , and E± rotational‐tunneling states and include the first observations of the c‐type Ka =1–0 band for the A±2 and B±2 states and the a‐type Ka =0–0 band for the A±1 states. For the A±1 states an interconversion tunneling splitting of 22.6 GHz is obtained, compared to the 19.5 GHz value found previously for the Ka =0 A±2 and B±2 states.

Vibrational, rotational, and tunneling dependence of vibrational predissociation in the HF dimer

Vibrational predissociation linewidths have been resolved in the two H–F stretching bands of the HF dimer using an optothermal (bolometer‐detected) molecular‐beam color‐center laser spectrometer. In addition to the strong vibrational mode dependence reported earlier by several groups, we observe a substantial K‐rotational and tunneling dependence to the longer‐lived mode ν1, which is associated with the ‘‘free‐H’’ stretch. The predissociation linewidths (FWHM in MHz) for this vibration are 6.4(5) for K=0+, 9.5(5) for K=0−, 10.2(5) for K=1+, and 11.8(5) for K=1−, where the +/− superscripts refer to the symmetric/antisymmetric tunneling states. The J dependence (at low J) is negligible compared to the K dependence. The K=0 levels of the ‘‘bound‐H’’ stretch have tunneling‐independent widths of 330(30) MHz. Extraneous broadening due to saturation effects was observed and corrected for in these measurements.