The gauche-trans Energy Difference of n-Butane from a Doppler Limited Investigation of the 740 cm- 1 CH2-Rocking Region

Tunable diode laser spectra of n-butane gas were recorded in the 740 cm -1 CH2-rocking region using path lengths of typically 300 m, pressures below 1 mbar and temperatures between 170 and 300 K. The spectra are complicated by the overlap of the two trans and gauche rotameric fundamentals, and by a strong absorption background probably caused by the addition of hot-band transitions involving excitation of one or more quanta of the torsion vibrations around the central C - C bond. Analysis was aided by Fourier transform recordings in medium and high resolution and lead to the assignment of rovibrational features close to the band centers at v0 (trans) = (733.5797 ±0.0015) cm -1 and v0(gauche) = (747.5116 ± 0.0018) cm-1 of the trans and gauche rotamers, respectively, and to medium accurate spectroscopic parameters. A statistical evaluation of the temperature dependence of the relative absorption intensities of near-by trans and gauche transitions resulted in the trans-gauche rotameric energy difference of ΔH = (246 ± 18) cm -1 or (2.94 + 0.21) kJ mol-1 . This value is lower than those from previous determinations but still indicates an interesting decrease of the gauche-trans energy difference in going from gas to the liquid state.

The Mechanism of Magnetically Tuned Singlet-Triplet Avoided Crossings in the Ã(1)A(2)-&Xtilde;(1)A(1) 4(1)(0) Band of Thioformaldehyde H(2)C&dbond;S

Magnetically tuned singlet-triplet perturbations in the 4(1)Ã(1)A(2)-2(1)3(1)ã(3)A(2) system of thioformaldehyde, found in ortho-rotational states (I = 1, the two hydrogen spins parallel) have been identified as being caused by vibronic spin-orbit coupling. This perturbation mechanism has been confirmed in several avoided crossings observed in this work for para states (I = 0, hydrogen spins antiparallel) which are much stronger. Parametrization of the theory has led to a quantitative understanding of the experimental frequency-field relations, and to an accurate prediction of the rovibrational energies of the triplet state. This in turn permitted the detection of about 100 Doppler-limited 2(1)3(1)ã(3)A(2)-0(0) &Xtilde;(1)A(1) rovibronic transitions which led into fine structure states. The combined data was then used to determine a set of rotational, fine, and hyperfine triplet-state parameters, the term value T(0)(2(1)3(1)ã(3)A(2)) = (16 685.385 +/- 0.002) cm(-1), and the spin-orbit vibronic singlet-triplet coupling constant, W(ST) = (0.0691 +/- 0.0016) cm(-1). A large number of frequency perturbations observed in the crossings, ranging from 2 to 300 MHz, can be explained with this single parameter. Copyright 2000 Academic Press.

Single-scan current-modulated tunable diode laser spectrometer of improved calibration and throughput performance

A novel version of a double-beam diode laser spectrometer has been built. Etalon decalibration is avoided, and the advantage of current modulation can also be utilized; both at the same time have not been possible hitherto. The monochromator optics were changed to enhance throughput energy. Signal processing is comparably simple and inexpensive.

Der Rotations-Zeeman-Effekt der l-Typ-Übergänge linearer Molekeln. OCS und HCN

The high-field rotational Zeeman effect has been observed in several rotational transitions of the (010) vibrational state in OCS and HCN. The magnetic-field splittings are in agreement with a simple first-order energy expression which is derived to hold for the Zeeman energies of rotation-vibrational states of a linear polyatomic molecule showing rotational l-type-doubling. In this way, the presence of intrinsic magnetic moments in the π-vibrational states has been shown experimentally. The g-values along the molecular axis are gǁ(010)= +0.061 ± 0.002 for OCS and gǁ(010)=±0.38 ± 0.06 for HCN. No magnetic anisotropics could be detected within the plane perpendicular to the molecular axis. The other parameters measured are g⊥(010)= -0.0285 ± 0.0006 and (χ⊥ - χǁ)(010) = (8.0 ±1.0) × 10-6 erg/G2mole for OCS and gǁ(010) = ∓ 0.100 ±0.001 for HCN which "can be considered an approximate value for the vibrational ground-state. Either the upper or the lower signs hold for the g-values of HCN. The intrinsic g-values, gǁ(010) , are discussed in terms of nuclear and electronic contributions. A quantity measuring the slip of a rotating nuclear framework within its electronic environment is defined and also discussed.