On the relationship betweenT 1 andT 2 for stochastic relaxation models

We consider the relaxation dynamics of two quantum levels coupled to a stochastic bath. We emphasize that even if the matrix elements of the fluctuating Hamiltonian are Gaussian, a second-order cumulant truncation is not exact. For various stochastic models, including the case of a spin-1/2 particle in a fluctuating magnetic field, we calculate 1/T ₁, the population relaxation rate, and 1/T ₂, the phase relaxation rate, up to fourth order in perturbation theory. We show that unlike the commonly accepted second-order result that 1/T ₂1/2T ₁, when fourth-order terms are included, in some instances 1/T ₂<1/2T ₁.     

Ultrafast vibrational spectroscopy of water and aqueous N-methylacetamide: Comparison of different electronic structure/molecular dynamics approaches

Kwac and Cho [J. Chem. Phys. 119, 2247 (2003)] have recently developed a combined electronic structure/molecular dynamics approach to vibrational spectroscopy in liquids. The method involves fitting ab initio vibrational frequencies for a solute in a cluster of solvent molecules to a linear combination of the electrostatic potentials on the solute atoms due to the charges on the solvent molecules. These authors applied their method to the N-methylacetamide-D/D(2)O system. We (S. A. Corcelli, C. P. Lawrence, and J. L. Skinner, [J. Chem. Phys. 120, 8107 (2004)]) have recently explored a closely related method, where instead of the electrostatic potential, the solute vibrational frequencies are fit to the components of the electric fields on the solute atoms due to the solvent molecules. We applied our method to the HOD/D(2)O and HOD/H(2)O systems. In order to make a direct comparison of these two approaches, in this paper we apply their method to the water system, and our method to the N-methylacetamide system. For the water system we find that the electric field method is superior to the potential approach, as judged by comparison with experiments for the absorption line shape. For the N-methylacetamide system the two methods are comparable.     

Thermochemistry of bis-arene- and arenetricarbonyl-chromium compounds containing hexamethylbenzene, 1,3,5-trimethylbenzene and naphthalene.

Microcalorimetric measurements at elevated temperatures of the heats of thermal decomposition and iodination have led to values of the standard enthalpies of formation of the following crystalline compounds (values given in kJ mol^?1) at 298K: [Cr(η⁶-1,3,5-C₆H₃(CH₃)₃)₂] = (63±12); [Cr(η⁶-C₆(CH₃)₆)₂] : -(88±12); [Cr(1,2,3,4,4a,8a-η-C₁₀H₈)₂] = (407±11); [Cr(CO)₃(1,2,3,4,4a,8a-η-C₁₀H₈)] = -(258±8). Separate measurements by the vacuum sublimation microcalorimetric technique gave the following values for the enthalpy of sublimation at 298K (kJ mol^?1) : [Cr(η⁶-1,3,5-C₆H₃(CH₃)₃)₂] = (104±1); [Cr(η⁶-C₆(CH₃)₆)₂] = (119±4); [Cr(CO)₃(1,2,3,4,4a,8a-η-C₁₀H₈)] = (107±3). From these and other data, the bond enthalpy contributions of the metal-ligand bonds in the gaseous metal complexes were evaluated as follows: [(η₆-C₆(CH₃)₆)-Cr] (155±7); [(η⁶-C₆H₃(CH₃)₃)-Cr] (151±6); [(1,2,3,4,4a, 8a-η-C₁₀H₈)-Cr](145±6) kJ mol^?1]The question of the transferability of the enthalpy contributions of chromium—ligand bonds between organochronium complexes is discussed with aid of information from structural and spectroscopic investigation. The limitations of the procedure are defined.The thermodynamic data are used to discuss various substitution, redistribution and exchange reaction of Cr(η-arene)₂ and [Cr(CO)₃(η-arene)] compounds.