A vibrational force field for pyridine has been determined by making use of ab initio calculated force constants as the starting point. These constants were adjusted to fit the observed frequencies for pyridine, pyridine-d5, and partially deuterated pyridines. The calculated frequencies allowed an assignment of the uncertain frequencies for the partially labeled compounds, and a constrained adjustment of the ab initio F matrix to give a "best fit" to the observed frequencies provided a fairly accurate force field. The infrared band intensities for pyridine and pyridine-^w ere determined in solution and were converted to dipole moment derivatives. The relationship between these quantities and the charge distribution in pyridine is discussed.We have been interested in the changes in properties produced by successive replacement of CH units in benzene by N, leading to the azines.1 The properties of the remaining CH units are of particular interest, and as a result we have wished to obtain

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The Fourier transform infrared absorption spectra of the C-H stretching region and the ull band of pyridine in a free jet expansion are presented. Also shown are the absorption spectra of pyridine and pyridine-d5 isolated in low-temperature matrices of inert gases. Comparison with the room temperature gas-phase spectra of these molecules has resolved some uncertainties concerning the assignments in the C-H stretching region in pyridine and has led to a revision of the assignments of the C-D fundamental stretching modes in pyridine-d5. The sequence bands and isotope peaks observed for uI1 of pyridine are analyzed by using predicted Boltzmann intensities and isotope shifts determined from a normal-coordinate analysis. The rotational and vibrational cooling of pyridine in a free jet are discussed.

Collisionfree lifetimes and quenching of the A 2Σ+ state of HS and DS radicals J. Chem. Phys. 79, 130 (1983); 10.1063/1.445572Fluorescence excitation spectra, lifetimes, and quenching of S2(B 3Σ u −) in the wavelength region 280-315 nm Pulsed laser excited tluorescence in the B 21: + +-X 21T system of gas phase PO was used to measure the lifetime for Vi = 0 of the B state. Rotationally resolved measurements for a few selected J I levels, at Ar or He carrier gas pressures of -2 Torr, reveal no dependence of the lifetime on the rotational level excited. Earlier measurements of relative tluorescence intensities in the v' = 0 vibrational progression were reinterpreted to extract the dependence of the electronic transition moment on internuclear distance. Using this transition moment, no lifetime dependence on rotational level is to be expected, even at low pressures. Rate constants for quenching oftheB state PO by N 2 , O 2 , CO 2 , and H 2 0, and upper limits thereoffor He and Ar are reported. O 2 was found to react with ground state PO. A crude measurement of the rate constant was performed. The result is compared to two other known measurements. The rate constant is in excellent agreement with the previous measurement, but in poor agreement with that of a concurrent study. Further work is required to determine the reason for the discrepancy. A cathode ray-vidicon type waveform digitizer was found to lend itself readily to the lifetime measurements, wherein a low repetition rate laser was used. Unfortunately, it was discovered that small nonlinearities ( -1 % offull scale) in the digitizer response have a marked effect on the fits to the exponential lifetime decays and, especially, the quenching rate constants. We believe this problem is not commonly recognized. A simple method for calibration of this type of digitizer and removal of these effects is presented. a valence state. IS Another study, however, concluded that this state is of at least partial Rydberg character. 17 Later calculations showed that a bound 21: + state would result if atomic Rydberg orbitals were included in the basis set. 18 However, Ghosh and Verma 3 (c) did not include th~ B state in their assignments of 21: + Rydberg states leading to 11: + PO+ . Moreover, because of their very diffuse nature, large

The laser excited fluorescence spectrum of NCO from an atmospheric pressure CH4/N2O/N2 flame is reinvestigated with a more sensitive apparatus than used previously. The NCO is pumped from the X 2Π (1,01,0) level to the A 2Σ+ (0, 00, 0) level. Emissions to (0, 01, 0)2Π, (1, 01, 0)2Π, (0, 01, 1)2Π, and (0, 10, 0)2Σ+ in the X state are observed. The previous tentative assignment of pumping transition as Q231 is confirmed. The confirmation of pumping transition solidifies our earlier interpretation that excited state spin relaxation in the flame is much faster than electronic quenching. In addition, analysis of the fluorescence spectra yields the first reported constants for the X(1, 01, 0)2Π level and spin splitting for X(0, 01, 1)2Π level in the gas phase.