This recommendation proposes a definition for the term "halogen bond", which designates a specific subset of the inter-and intramolecular interactions involving a halogen atom in a molecular entity.

Analytical representations of the global potential energy surface of XYn molecules are developed and applied to model the potential surface of methane in the electronic ground state. The generic analytical representation allows for a compact, robust, and flexible description of potentials for XYn systems irrespective of the specific nature of the atomic interactions. The functions are global in that structures near several minima of the potential hypersurface as well as saddle points and dissociation limits are well described. Clusters of atoms Yn can be represented as well by this type of function. Care is taken to implement conditions resulting from the symmetric group Sn and to construct positive definite bilinear forms of special functional forms of certain coordinates (such as bond lengths and bond angles), in order to avoid artifacts in exceptional ranges of the potential hypersurface. These special functional forms include intrinsic, symmetry allowed couplings between coordinates such as bending and stretching. We include linear potential terms in bond angle coordinates, which result in effectively quadratic potential terms for highly symmetric structures. True logical multidimensional 01-switching functions Ssw(r) of bond lengths r are used to interpolate between limiting ranges in the hypersurface. The particular form Ssw(r)∼exp(−(rsw/r)nsw) allows us to describe the potential as a multipole expansion representation in the limit of large r(→∞). In the application to methane, first the representations are fitted to data from high level ab initio calculations using multireference configuration interaction techniques. Additional conditions which help to improve the description of experimental data are considered during the fit. Typically, these conditions involve some parameters or parameter groups and refer to the equilibrium geometry and harmonic force field. Other constraints apply to the energies of dissociation channels. We describe the model potentials METPOT 1 to METPOT 4 in the present work.

Articles you may be interested in Energetics and dipole moment of transition metal monoxides by quantum Monte CarloA full nine-dimensional potential-energy surface for hydrogen molecule-water collisions A nine-dimensional perturbative treatment of the vibrations of methane and its isotopomersThe pure rotational spectrum in the far-infrared and its absolute intensity in the vibrational ground state of CHD 3 and CH 3 D, and the integrated band strength of the N=5 CH-stretching overtone of CHD 3 in the near infrared to visible were measured by high-resolution interferometric Fourier transform techniques. The far-infrared data result in permanent electric dipole moments (liLbl=(5.69±O.14)XIO-3 D for CHD 3 , 1fL51=(5.57±O.10)XIO-~ D for CH 3 D), consistent with previous experimental data. The integrated N = 5 overtone cross section is found to be (O.828±O.068) fm2. The overtone data are used, together with previous data, to derive a new, nine-dimensional, isotopically invariant dipole moment function for CH 4 within the cbromophore model for the CH cbromoph,ore in CHD 3 . With this function, the experimental data can be reproduced to an averaged factor of 1.2, in the best case. In the vibrational ground state, a nine-dimensional calculation of expectation values on a new, fully anharmonic potential surface was performed using the solution of the rovibrational Schrodinger equation by diffusion quantum Monte Carlo methods. The results for the rotational constants of several isotopomers, which include significant contributions from rovibrational interactions, indicate that the eqUilibrium CH bond length of methane is r e = 108.6 pm. The calculated value for the vibrationally averaged permanent . dipole moment from these nine-dimensional vibrational quantum calculations, using the dipole moment function consistent with the analysis of the overtone bands, is fL5= -(6.6±0.4)XIO-3 D for CHD 3 (with positive z coordinate for the H atom) and ,u~=(6.8±O.5)XIO-3 D forCH 3 D (with positive z coordinate for the D atom) in essential agreement with the far-infrared rotational intensities. The sign could be determined unambiguously by comparison with ab initio data. We predict the permanent dipole moment of several further methane isotopomers. The polarity of the CH bond in methane is C--H+, within our simple bond dipole model, but is discussed to be a model dependent (not purely experimental) quantity.3588

The global analytical potential surface for the electronic ground state of methane developed in paper I is analyzed and discussed in detail. A new determination of the experimental potential surface for the CH chromophore in CHD 3 , obtained from more recently measured line positions and integrated absorption coefficients, is also reported. The complete, nine-dimensional calculation of the vibrational ground state by diffusion quantum Monte Carlo on the fully anharmonic potential surface allows the determination of r e ) (1.086 ( 0.002) Å with a high level of certainty from comparison with experimental values of rotational constants for methane and isotopomers. Other results regarding properties of the anharmonic potential surface close to the equilibrium configuration are theoretical values for the vibrationally induced electric dipole moments in CH 3 D, CH 2 D 2 , and CHD 3 , which are obtained in conjunction with a nine-dimensional, vector-valued representation of the electric dipole moment in this molecule and agree well with the experimental data. It is shown that, if equilibrium geometry and harmonic force field are fixed to experimental values, the overtone spectrum of the CH chromophore in CHD 3 can be described in an acceptable way (〈|ν cal -ν obs |〉 ≈ 40 cm -1 up to 18 000 cm -1 (METPOT 3). The agreement can be improved to within 17 cm -1 (METPOT 4), on the average, if the anharmonic part of the model potential is refined with data from the experimentally derived, three-dimensional CH chromophore potential surface from Lewerenz and Quack (J. Chem. Phys. 88). For this purpose, the analytical representation of the potential, mainly along the bending degrees of freedom, must be sufficiently flexible, as shown by the present calculations. The accuracy regarding the description of spectroscopic data pertaining to highly excited vibrational states and the global character of the proposed potential surface representation render it a powerful instrument for the theoretical treatment of chemical reaction dynamics. A relation to reaction kinetics can be established through calculation of the lowest adiabatic channel on the complete nine-dimensional potential hypersurface for methane using quasiadiabatic channel quantum Monte Carlo techniques. It is found that the behavior of this channel, corresponding approximately to an exponential interpolation with a parameter R ≈ 0.7-0.8 Å -1 in the adiabatic channel model, is consistent with empirical results obtained from experiment. Further refinements of the models are feasible and expected, when full dimensional calculations of the solution of the rovibrational Schrödinger equation will be performed.

An analytical, full-dimensional, and global representation of the potential energy surface of NH(3) in the lowest adiabatic electronic state is developed, and parameters are determined by adjustment to ab initio data and thermochemical data for several low-lying dissociation channels. The electronic structure is calculated at the CASPT2 level within an [8,7] active space. The representation is compared to other recently published potential energy surfaces for this molecule. The present representation is distinguished by giving a qualitatively correct description of the potential energy for very large amplitude displacements of the nuclei from equilibrium. Other characteristic features of the present surface are the equilibrium geometries r(eq)(NH(3)) approximately 101.24 pm, r(eq)(NH(2)) approximately 102.60 pm, alpha(eq)(NH(3)) approximately 106.67 degrees, and the inversion barrier at r(inv)(NH(3)) approximately 99.80 pm and 1781 cm(-1) above the NH(3) minimum. The barrier to linearity in NH(2) is 11,914 cm(-1) above the NH(2)((2)B(1)) minimum. While the quartic force field for NH(3) from the present representation is significantly different from that of the other potential energy surfaces, the vibrational structures obtained from perturbation theory are quite similar for all representations studied here.

The quantum vibrational dynamics of the CH-chromophore in a chiral environment are studied with the examples CHDTMu, CHDTF and CHFClBr. For the chiral methane isotopomer we use a recently established nine-dimensional potential hypersurface to extract the three-dimensional short-time quantum dynamics and the related CH-overtone spectra. We have carried out ab initio (MP2) calculations in the appropriate normal coordinate subspace for CHDTF, a chiral isotopomer of methylfluoride for which we have previously carried out extensive related calculations, and also experimental investigations on other isotopomers. For CHFClBr we report the first experimental and theoretical study of the CH-chromophore overtone spectra. The results are systematically analysed in terms of anharmonic coupling constants of the effective hamiltonian as well as the potential hypersurfaces in the appropriate three-dimensional subspaces. We show that the chiral, symmetry-breaking coupling constant ksab is of appreciable absolute magnitude for all three cases (ca. 25 cm-' for CHFClBr). The resulting fast intramolecular vibrational redistribution in the highly excited CHchromophore, on the femtosecond timescale, leads to appreciable population transfer between states of dynamical a' and a" symmetry for the electronically, 'chemically' chiral CHFClBr and for CHDTF, which is chiral only by isotope substitution. The symmetry-breaking intra-molecular redistribution processes in chiral molecules are briefly discussed in relation to dynamical chirality, time-dependent optical activity and fundamental symmetry violations of parity, and even of charge conjugation, parity and timereversal symmetry and their combinations.

We report results from quantum dynamical simulations of ultrafast vibrational redistribution processes in the CH chromophore of CHX3 molecules (CHD3, CHF3) during and after infrared-multiphoton excitation. The vibrational Hamiltonian is based on results from high resolution spectroscopy and ab initio calculations of the potential hypersurfaces for these molecules. The quantum dynamical calculations involve accurate solutions of the time dependent quantum equations of motion by means of both Floquet and quasiresonant approximations. We find mode selective redistribution between the CH stretching and bending modes on a time scale of 50 to 100 fs. Other modes participate only on much longer time scales (>1 ps), as was shown previously by analysis of the spectra. For the real, strongly anharmonic systems (k′sbb≂30 to 100 cm−1 ), the redistribution is nonclassical with fast spreading to a quasimicrocanonical distribution, which is particularly pronounced if a narrow range of energies (for example, the N=6 polyad) is initially excited. The effect can be interpreted as an intrinsic quantum statistical behavior induced by anharmonicity. In comparison, a weakly anharmonic hypothetical model system (ksbb≤2 cm−1) leads to quasiclassical motion of the wave packet with quasiperiodic exchange between stretching and bending motions. We present an approximate analytical investigation of the Fermi modes underlying the dynamics which provides a semiquantitative understanding of the Fermi-resonance spectra. On the basis of these results, we discuss possibilities of mode selective reaction control in unimolecular processes with laser excitation and some aspects of intramolecular ‘‘chaos.’’

The time dependent quantum dynamics of the large amplitude motion of the NH stretching chromophore in NHD2 is investigated during and after coherent multiphoton excitation by calculation of the wave packet evolution using global analytical potential energy and electric dipole hypersurfaces of ammonia derived from ab initio calculations. Intramolecular vibrational redistribution between the NH stretching and bending motion and coupling to the radiation field induces a diffusion of probability density into the NH chromophore space, which includes the inversion coordinate. However, inversion remains essentially dominated by a tunneling process, even at average energies well above the inversion barrier.