摘要:

The acceptance ratio method is adapted for the determination of free energy of molecular systems with several local minima in the potential energy function. The procedure uses configurations from Monte Carlo simulations. A special reference system, of which the free energy can be computed analytically, is constructed in the process. Two variants of the technique are tested on a simple model of a macromolecule. In this simple test the method turned out to be more efficient than thermodynamic integration.

ISSN:0026-8976    

DOI:10.1080/00268979300101841

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The temperature dependencies of two fast vibrational energy transfer processes in methyl fluoride (CH3F) have been measured between 120K and 400 K by means of time-resolved millimetre/submillimetre-infrared double resonance spectroscopy. The first of these processes, a resonant vibrational swapping process between the ground vibrational state and thev3= 1 (v3) vibrational state, effectively transfers population between states of A and E symmetry. A rapid increase in cross section with decreasing temperature was observed for this process, a result in excellent quantitative agreement with semiclassical theory of near resonant vibrational collisions. The second process, which transfers population between thev3andv6= 1 (v6) vibrational states, was found experimentally to have a much weaker temperature dependence. From this result and from additional experimental observations of symmetry type-sensitive energy transfer intov6, the energy transfer betweenv3andv6was demonstrated to result from a ‘direct’ vibrational energy transfer rather than from an ‘indirect’ vibrational swap process.

ISSN:0026-8976    

DOI:10.1080/00268979300101851

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In this paper we report experimental and theoretical results on total inelastic cross-sections for NH3molecules colliding with H2at room temperature. The use of time resolved infrared-infrared double resonance allows one to follow the population relaxation for different energy levels of NH3. A CO2pulsed pump laser tuned to a line in thev2band of NH3induces a transient transfer of population and the return to equilibrium is probed with a c.w. diode laser turned to lines in thev4band of NH3. The time evolution of the probe absorption for different pressures of the two gases is then related to the inelastic cross-sections of the states involved. The so-determined cross-sections for the (j= 5,k= 4, ε = +1) and (j= 1,k= 1, ε = -1) states of NH3are respectively 27 Å2and 31 Å2. We have also calculated inelastic cross-sections using a semi-classical model and anab initiopotential energy surface for different values of (j, k, ε) (i.e.j= 1, 2, 3,k= 1, 2, 3 and ε = ±1) and ofj2(total angular momentum of H2). These calculated values are in agreement with the experimental results.

ISSN:0026-8976    

DOI:10.1080/00268979300101861

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The rotation-vibration spectrum of the weakly bound complex Ne-CO has been studied in the 2140 cm-1region of the fundamental stretching vibration of carbon monoxide. Spectra were obtained using two different and complementary techniques: first, a pulsed supersonic jet expansion and a tunable infrared diode laser source; and, second, a long-path, low-temperature, static gas cell and a Fourier transform infrared spectrometer. The spectrum of Ne-CO is approximately that of a T-shaped asymmetric rotor, but the complex undergoes very large amplitude intermolecular bending and stretching motions. About 106 transitions for20Ne-12C16O, and 88 for22Ne-12C16O, were assigned to the perpendicular subbands withK= 1 ← 0 and 0 ← 1 and to the parallel subband withK= 0 ← 0. Tentative observations were also made of theK= 2 ← 1 and 1 ← 2 subband centres, and of a bending combination band of the20Ne isotope. The observedB¯rotational constant in theK= 0 ground state was 0·10873 cm-1for20Ne-CO, which corresponds to an effective intermolecular separation of 3·646 Å. The observedArotational constant of about 3·57 cm-1approaches the value that would be expected in the limit of free CO internal rotation, namely 2Bfor CO or 3·84 cm-1.

ISSN:0026-8976    

DOI:10.1080/00268979300101871

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Laser-excitation and dispersed fluorescence spectra of the CF3CO radical are reported. The 0-0 band of the Ã-[Xtilde] transition lies at 26 023 cm-1(384·28 nm). The spectra are characterized by a prominent progression involving the CO stretching mode. The fundamental CO stretching frequency in the ground state is 1525 cm-1and compares well with the CO stretching mode of 1574 cm-1observed for CF3CO by Kozuka and Isobe. The CO stretching mode of the excited state is reported as 818 cm-1. The radiative lifetime of the excited state is 0·34 ± 0·01 μs. The argon quenching rate constant for the excited state is determined as (6·0 ± 0·20) × 105Torr-1s-1. The CF3CO radical is found in the infrared multiphoton dissociation of CF3C(O)Cl, as well as in the CF3+ CO reaction.

ISSN:0026-8976    

DOI:10.1080/00268979300101881

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Therαstructure of thietane has been obtained from its previously measured dipolar couplings. The molecular geometry can be described by a model of hindered pseudorotation characterized by a double minimum potential function. The optimized values for the geometric parameters are:rCH= 1·077 Å, HCαH = 109·7°, HCβH = 108·8° and -6·8° for the bending coordinate. The puckering amplitudeqcorresponding to the potential minimum is 0·24Å, but the height of the inversion barrier (δV) cannot be determined because the surface σ(q, δV) for the root mean square (RMS) deviations between observed and calculated dipolar couplings shows a valley of minimum deviation instead of local minima.

ISSN:0026-8976    

DOI:10.1080/00268979300101891

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

For closed shells in a bare Coulomb field, the one-centre densityp(r) can be characterized solely by s states. Here, the two-centre bare Coulomb field is considered for the molecular ion H+2. The potential energy curve, known to be a unique function of the ground-state densityp(rR), withRthe internuclear separation, is shown here to be determined solely by the electron density on the plane perpendicularly bisecting the bond axis.

ISSN:0026-8976    

DOI:10.1080/00268979300101901

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor