A unified perspective on the nature of bonding in pairwise interatomic interactions.

Different classes of ground electronic state pairwise interatomic interactions are referenced to a single canonical potential using explicit transformations. These approaches have been applied to diatomic molecules N2, CO, H2(+), H2, HF, LiH, Mg2, Ca2, O2, the argon dimer, and one-dimensional cuts through multidimensional potentials of OC-HBr, OC-HF, OC-HCCH, OC-HCN, OC-HCl, OC-HI, OC-BrCl, and OC-Cl2 using accurate semiempirically determined interatomic Rydberg-Klein-Rees (RKR) and morphed intermolecular potentials. Different bonding categories are represented in these systems, which vary from van der Waals, halogen bonding, and hydrogen bonding to strongly bound covalent molecules with binding energies covering 3 orders of magnitude from 84.5 to 89,600.6 cm(-1) in ground state dissociation energies. Such approaches were then utilized to give a unified perspective on the nature of bonding in the whole range of diatomic and intermolecular interactions investigated.

Paired hydrogen bonds in the hydrogen halide homodimer (HI)2.

The HI homodimer was found to have structural and vibrational properties unlike any other previously studied (HX)(2) system, with X = F, Cl, and Br. The infrared spectrum of (HI)(2) is also observed to be distinctly different from the other members of the series. In addition, the interaction energy of the (HI)(2) dimer has been calculated using the coupled-cluster with singles, doubles, and perturbative triples [CCSD(T)] level of theory. A four-dimensional morphed intermolecular potential has been generated and then morphed using available near infrared and submillimeter spectroscopic data recorded in supersonic jet expansions. The morphed potential is found to have a single global minimum with a symmetric structure having C(2h) symmetry. The equilibrium dissociation energy is found to be 359 cm(-1) with the geometry in Jacobi coordinates of R(e) = 4.35 Å, θ(1) = 43°, θ(2) = 137°, and φ = 180°. The infrared spectrum is characterized by pairs of excited vibrational states resulting from the coupling of the two HI stretching modes. A qualitative model using a quadratic approximation has been fitted to obtain an estimate of this coupling. Furthermore, a morphed intermolecular potential for the vibrationally excited system was also obtained that gives a quantitative estimate of the shift in the potential due to the excitation. The submillimeter analysis is consistent with a ground state having its highest probability as a paired hydrogen bond configuration with R(0) = 4.56372(1) Å and an average angle θ=cos(-1)((1/2)) = 46.40(1)° (between the diatom center of mass∕center of mass axis and direction of each component hydrogen iodide molecule). On monodeuteration, however, the ground state is predicted to undergo an anomalous structural isotope change to an L-shaped HI-DI structure with highest probability at R(0) = 4.51 Å, θ(1) = 83°, θ(2) = 177°, and φ = 180°. These results provide a test for large scale ab initio calculations and have implications for the interpretation of photoinduced chemistry and other properties of the dimer.

Improved morphed potentials for Ar-HBr including scaling to the experimentally determined dissociation energy.

A lead salt diode infrared laser spectrometer has been employed to investigate the rotational predissociation in Ar-HBr for transitions up to J' = 79 in the v(1) HBr stretching vibration of the complex using a slit jet and static gas phase. Line-shape analysis and modeling of the predissociation lifetimes have been used to determine a ground-state dissociation energy D(0) of 130(1) cm(-1). In addition, potential energy surfaces based on ab initio calculations are scaled, shifted, and dilated to generate three-dimensional morphed potentials for Ar-HBr that reproduce the measured value of D(0) and that have predictive capabilities for spectroscopic data with nearly experimental uncertainty. Such calculations also provide a basis for making a comprehensive comparison of the different morphed potentials generated using the methodologies applied.