The rotational spectra, electric dipole moments and molecular structures of anisole and benzaldehyde.

The rotational spectra of anisole and of benzaldehyde were investigated in supersonic expansion at frequencies up to 41 GHz, and at room temperature in the millimetre-wave region, from 170 to 330 GHz. Accurate spectroscopic constants for the parent isotopomers in the ground vibrational state and for the first excited torsional state were determined for both molecules. The supersonic expansion spectrum allowed measurement, in natural abundance, of all singly substituted 13C isotopomers, as well as of the 18O isotopomer for both anisole and benzaldehyde. The rotational constants were used to determine the r(s) and the r(m)(1) gas-phase geometries, which are found to be consistent with prediction of bond length alternation in the phenyl ring induced by the asymmetric substituent. Stark measurements were made on the supersonic expansion spectrum resulting in electric dipole moment determination, /mu a/ = 2.9061(22) D, /mu b/ = 1.1883(10) D, /mu tOt/ = 3.1397(24) D for benzaldehyde and /mu a/ = 0.6937(12) D, /mu b/ = 1.0547(8) D, mu tOt = 1.2623(14) D for anisole. During the investigation it was found that use of a carrier gas mixture consisting of 30% Ar in He carries significant advantages for studies of weak lines, and pertinent experimental details are reported.

The Gas-Phase Electric Dipole Moments of the Symmetric Top Tertiary Butyl Molecules (t)BuX, X=F, Cl, Br, I, CN, and NC.

The electric dipole moments of symmetric top tertiary butyl derivatives were determined from Stark effect measurements made with cavity Fourier transform microwave spectroscopy under conditions of supersonic expansion. The resulting values are 1.9562(15), 2.1817(16), 2.2574(17) and 2.2122(17) D for tertiary butyl fluoride, chloride, bromide, and iodide, and 4.0129(30), and 4.0640(31) for tertiary butyl cyanide and isocyanide, respectively. The experimental values are compared with ab initio calculations and with experimental and calculated dipole moments for the corresponding methyl derivatives. Copyright 2001 Academic Press.

The angleICI Bending Satellites in the Millimeter-Wave Rotational Spectra of CH(2)I(2) and CD(2)I(2).

Rotational transitions in the first four excited states of the low-frequency angleICI bending mode, nu(4), have been assigned in the mm-wave rotational spectra of CH(2)I(2) and of CD(2)I(2). Measurements of transition frequencies, made over the frequency region 167-326 GHz and for J" up to 190, allowed determination of sextic level spectroscopic constants for all states. The changes in spectroscopic constants with vibrational excitation show very small anharmonicity, in spite of the very low frequency of this mode (121 cm(-1)). Vibrational excitation affects the moments of inertia in such a way that the planar moment P(b), about the plane perpendicular to both angleICI and angleHCH, is practically invariant. Vibrational change in P(c), the moment along the principal axis in the HCH plane and perpendicular to the angleHCH bisector, has been successfully reproduced with an ab initio harmonic force field so that there is no discernible vibrational change in angleHCH on excitation of angleICI. Finally, the change in P(a) leads to estimated vibrational change of +0.12 degrees in the value of angleICI itself. Copyright 2000 Academic Press.

Investigation of the Rotational Spectrum of Pyrimidine from 3 to 337 GHz: Molecular Structure, Nuclear Quadrupole Coupling, and Vibrational Satellites.

A comprehensive reinvestigation of the rotational spectrum of pyrimidine was carried out by using several different spectrometers. All singly substituted 13C- and 15N-isotopic species of pyrimidine have been measured in natural abundance with millimeter-wave free jet and waveguide Fourier transform microwave techniques, and complete rs and r0 heavy atom geometries have been determined. The ground state rotational spectrum in the centimeter-wave region was measured at sub-Doppler resolution of the cavity Fourier transform spectrometer and all elements in the inertial and principal nuclear quadrupole-coupling tensors of the nitrogen nuclei in pyrimidine have been determined. The room-temperature spectrum was measured up to 337 GHz and J = 66 with BWO-based spectrometers and sextic level centrifugal distortion constants in the rotational Hamiltonian have been determined for the ground state and three lowest vibrational fundamentals of pyrimidine. Copyright 1999 Academic Press.

Rotational Spectrum of CD2I2

The rotational spectrum of CD2I2 was measured and analyzed by combining results from two different millimeter wave spectrometers. Low-J transitions were measured with a free jet spectrometer at conditions of completely resolved hyperfine structure over the frequency range 59-69 GHz. High-J rotational transitions were measured at room temperature at frequencies 167-338 GHz and for J" up to 190". Spectroscopic constants in the sextic rotational Hamiltonian of CD2I2 have been determined, and the analysis of the observed hyperfine structure yielded all components in the inertial and in the principal nuclear quadrupole coupling tensors for the iodine nuclei. The rotational constants for CD2I2 and those for CH2I2 from Z. Kisiel, L. Pszczólkowski, W. Caminati, and P. G. Favero (1996. J. Chem. Phys. 105, 1778-1785) have been used to evaluate a full r0 structure and have been combined with the results of ab initio calculations to also evaluate the average r* structure: r(CI) = 2.1364 Å, r(CH) = 1.078 Å, <(ICI) = 113.83 degrees, and <(HCH) = 113.3 degrees. The current results complete the first analysis of a rotational spectrum of a molecule containing two iodine nuclei. Copyright 1998 Academic Press.

The Rotational Spectrum of CBrClF2 (Halon BCF): II. The Lowest Excited Vibrational States and Nuclear Quadrupole Coupling Tensors

The analysis of the millimeter-wave rotational spectrum of the well-known halon molecule CBrClF2 was extended to rotational transitions in the excited vibrational states v9 = 1 (307 cm-1) and v5 = 1 (337 cm-1). The two states are appreciably coupled by a- and b-axis Coriolis interaction which was treated explicitly and spectroscopic constants were determined from measurements at frequencies from 121 to 330 GHz and J" from 18 to 111. The present work, together with our previous study (1996. J. Mol. Spectrosc. 177, 240-250), results in accurate constants in the rotational Hamiltonian for the four lowest vibrational states of C79Br35ClF2 and C81Br35ClF2. The double nucleus hyperfine structure in low-J transitions of the two 35Cl isotopomers of CBrClF2 was also measured in the region 3-14 GHz with a supersonic beam, cavity Fourier transform microwave spectrometer. All constants in both the inertial and the principal nuclear quadrupole coupling tensors have been determined for both Br and Cl nuclei. Copyright 1997 Academic Press. Copyright 1997Academic Press