René Wugt Larsen

Studies of intermolecular vibrations in hydrogen-bonded molecular complexes for the gas phase using a synchrotron radiation source

An electron storage ring acts as an approximate point source in the far-infrared spectral region with a much greater intensity than provided by conventional black body radiators. Far-infrared synchrotron radiation is therefore suitable for high-resolution spectroscopic measurements of low-frequency vibrations for metastable molecules such as the intermolecular vibrational modes of hydrogen-bonded molecular complexes. The present paper describes the use of synchrotron radiation from the MAX-I electron storage ring, Lund, Sweden, for far-infrared measurements of several hydrogen-bonded molecular complexes in the gas phase.

Microwave, High-Resolution Infrared, and Quantum Chemical Investigations of CHBrF2: Ground and v4 = 1 States

A combined microwave, infrared, and computational investigation of CHBrF(2) is reported. For the vibrational ground state, measurements in the millimeter- and sub-millimeter-wave regions for CH(79)BrF(2) and CH(81)BrF(2) provided rotational and centrifugal-distortion constants up to the sextic terms as well as the hyperfine parameters (quadrupole-coupling and spin-rotation interaction constants) of the bromine nucleus. The determination of the latter was made possible by recording of spectra at sub-Doppler resolution, achieved by means of the Lamb-dip technique, and supporting the spectra analysis by high-level quantum chemical calculations at the coupled-cluster level. In this context, the importance of relativistic effects, which are of the order of 6.5% and included in the present work using second-order direct perturbation theory, needs to be emphasized for accurate predictions of the bromine quadrupole-coupling constants. The infrared measurements focused on the ν(4) fundamental band of CH(79)BrF(2). Fourier transform investigations using a synchrotron radiation source provided the necessary resolution for the observation and analysis of the rotational structure. The spectroscopic parameters of the v(4) = 1 state were found to be close to those of the vibrational ground state, indicating that the ν(4) band is essentially unaffected by perturbations.

Observation and assignment of the hot band nu(2)+nu(1)(9)-nu(1)(9) in (HCN)(2)

The high-resolution infrared absorption spectrum for (HCN)(2) in the spectral region 3100-3300 cm(-1) is recorded in a static multipass cell at 240 K under equilibrium conditions. The spectrum shows extensive rovibrational structures which are due to the bound C-H stretching vibration band nu(2) of (HCN)(2) and several hot bands associated to nu(2). We manage to assign 63 new rovibrational transitions [P(97) to R(98)] associated with the nu(2) band leading to an extended analysis of the nu(2) band. This enables the assignment of 60 rovibrational transitions associated with the hot band nu(2) + nu(9)(1) - nu(9)(1) in (HCN)(2) together with the first rovibrational analysis of this hot band. The determined positive value of the anharmonicity constant X-2,X-9 of 0.2569(13) cm(-1) indicates that the intermolecular low-frequency bending vibration nu(9)(1) increases in frequency upon simultaneous excitation of the nu(2) mode. This result confirms that the HCN dimer is stiffer and more tightly bound upon vibrational excitation of the bound C-H stretching mode. (Less)