Anne M. Andrews

Infrared diode‐laser spectra of the ν9 and ν11 N–O stretching bands of N2O4

The rotationally resolved jet‐cooled infrared spectra of the b‐type ν9 (b2u) fundamental band at 1757 cm−1 and the a‐type ν11 (b3u) fundamental band at 1261 cm−1 of the N–O stretches of N2O4 have been recorded with a diode laser. The ν9 band was found to be unperturbed, and it was possible to assign nearly 100% of the observed lines with a signal to noise greater than 2. In contrast, most of the Ka states of the ν11 band were found to be strongly perturbed. A large number of strong lines (≊20%) are unassigned and presumably arise from the perturbing state as well as torsional hot band transitions. The rotational analysis yields precise spectroscopic constants for the ground vibrational state which are interpreted in terms of a planar centrosymmetric dimer with a N–N bond length of 1.756(10) A. The observed nuclear‐spin statistical weights and near‐zero inertial defect are consistent with the planar centrosymmetric structure determined in earlier electron‐diffraction studies.

Molecular-beam infrared–infrared double-resonance spectroscopy study of the vibrational dynamics of the acetylenic C–H stretch of propargyl amine

The acetylenic C–H stretch spectrum of propargyl amine near 3330 cm−1 has been measured at 0.0002 cm−1 (6 MHz) resolution with a tunable color-center laser in an electric-resonance optothermal spectrometer. The spectrum has been fully assigned through IR–IR double resonance measurements employing a tunable, microwave sideband-CO2 laser. The 10 μm spectrum of propargyl amine displays splittings in the two nuclear spin symmetry states arising from amino-proton interchange, allowing double-resonance assignment of the –NH2 group resultant proton nuclear spin quantum number in the highly fragmented 3 μm spectrum. The experimental state density is consistent with a (2J+1) increase that is expected if all near-resonant states are coupled. From this J-dependent growth in the state density we determine the density of states at J=0 to be 22 states/cm−1. This value is in reasonable agreement with the direct state count result of 16 states/cm−1. The unperturbed transition frequencies for the two different nuclear spi...

Study of the Overtone C-O Stretching Band of Methanol by Multiple Resonance Spectroscopy

Two microwave‐sideband CO2 lasers have been used with a molecular‐beam electric‐resonance spectrometer to study the overtone C–O stretching vibration of methanol. Infrared‐infrared double‐resonance results have been obtained for levels involving the K=1 and 2, A symmetry, and the K=2, E2 symmetry species. In the A torsional symmetry case, radio frequency‐infrared multiple resonance was used to obtain accurate asymmetry splittings for the vco=1 and 2, C–O stretching states. The asymmetry splitting constants determined for these states are in good agreement with the literature values for the first excited C–O stretching states. However, the nearly factor‐of‐2 change in the K=2 asymmetry splitting constant for the vco=2 level compared to the vco=0 and 1 level results suggests that this state is weakly perturbed. The overtone transition frequencies obtained in this work were combined with previous overtone Fourier‐transform results in a global fit to a torsion–rotation Hamiltonian to refine the fundamental mo...

ROTATIONAL SPECTRUM OF A DARK STATE IN 2-FLUOROETHANOL USING MICROWAVE/RADIO-FREQUENCY-INFRARED MULTIPLE RESONANCE

Microwave/radio‐frequency‐infrared multiple resonance has been used with an electric‐resonance optothermal spectrometer to characterize a weak 21.6 MHz perturbation in the infrared spectrum of the ν14 C–O stretching vibration of 2‐fluoroethanol. The infrared spectrum of 2‐fluoroethanol was recorded at a resolution of ∼2 MHz using a tunable microwave‐sideband CO2 laser. The spectrum is fit by an asymmetric‐rotor Hamiltonian to a precision of 0.6 MHz, except for the transitions to the 413 upper state which are split into doublets by an interaction between the 413 level and a rotational level of a nearby background, or dark, vibrational state. Microwave/radio‐frequency‐infrared double and triple resonance reveals that the 413 level of the C–O stretching vibration is interacting with the 431 level of the dark state. The rotational constants determined for the dark state allow us to assign the perturbing state to the ν18+4ν21 combination vibration of the lowest energy conformer, where ν18 is the CCO bending vi...

Fourier-transform infrared and jet-cooled diode-laser spectra of the 867 cm-1 v 9 band of acetaldehyde

The spectrum of the v 9 A′ C-C stretching fundamental band of acetaldehyde (CH3CHO) at 867 cm-1 has been recorded in a slit-jet nozzle expansion using a diode-laser spectrometer and in a room-temperature gas cell using a Fourier-transform infrared spectrometer. The spectrum exhibits a-type transitions which have been assigned for J′ ⩽ 1>9 and K′a ⩽ 5> for the A state and J′ ⩽ 1>9 and K′a ⩽ 4> for the E state. The transitions show a large tunnelling splitting of ≈0·5 cm-1 for K a = 0, attributed primarily to an anharmonic interaction of the v 9 vibration with the v 14 + v 15 torsional combination vibration at 920 cm-1. Evidence is also presented for an additional interacting vibration(s) affecting the v 9 tunnelling splitting.

Microwave spectrum, large‐amplitude motions, and ab initio calculations for N2O5

The rotational spectrum of dinitrogen pentoxide (N2O5) has been investigated between 8 to 25 GHz at a rotational temperature of ∼2.5 K using a pulsed‐molecular‐beam Fourier‐transform microwave spectrometer. Two weak b‐dipole spectra are observed for two internal‐rotor states of the molecule, with each spectrum poorly characterized by an asymmetric‐rotor Hamiltonian. The observation of only b‐type transitions is consistent with the earlier electron‐diffraction results of McClelland et al. [J. Am. Chem. Soc. 105, 3789 (1983)] which give a C2 symmetry molecule with the b inertial axis coincident with the C2 axis. Analysis of the 14N nuclear hyperfine structure demonstrates that the two nitrogen nuclei occupy either structurally equivalent positions or are interchanging inequivalent structural positions via tunneling or internal rotation at a rate larger than ∼1 MHz. For the two internal rotor states, rotational levels with Ka+Kc even have IN=0, 2, while levels with Ka+Kc odd have IN=1, where IN is the result...

14 – Microwave Spectroscopy

This chapter discusses the state-of-the-art techniques being used in conventional microwave spectroscopy today, as well as explores three new molecular beam microwave techniques that are the current hotbed of activity in the field. It is these latter techniques that are revolutionizing the field once again by allowing the study of species that would have been considered much too difficult a decade ago. The cooling effect attained by the rapid expansion of the molecular beams in these spectrometers greatly simplifies the rotational spectra of large species, resulting in spectra that are readily assignable because only the lowest energy levels of the species are populated. In addition, the expansion process permits species in the molecular beam to cluster with the carrier gas or other species in the beam; the spectra of these clusters provide structural information about van der Waals complexes and hydrogen-bonded dimers, trimers, and tetramers.