ARTICLE pubs.acs.org/JPCA
Probing the Dependence of Long-Range, Four-Atom Interactions on Intermolecular Orientation: 3. Hydrogen and Iodine Joshua P. Darr† and Richard A. Loomis* Department of Chemistry, Washington University—St. Louis, One Brookings Drive, CB 1134 Saint Louis, Missouri 63130, United States
Sara E. Ray-Helmus and Anne B. McCoy* Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, United States ABSTRACT: Two-laser, action spectroscopy experiments have been performed in the I2 BX, υ0 0 spectral region on H2 3 3 3 I2 and D2 3 3 3 I2 complexes to investigate the dependence of the H2/D2 þ I2 intermolecular interactions on orientation. The spectra contain features associated with at least two different conformers of the ground-state H2/D2 3 3 3 I2(X,υ00 = 0) complexes; one conformer has a preferred T-shaped geometry with the H2/D2 moiety localized in a potential minimum that is orthogonal to the I—I bond axis, and the second conformer has a linear geometry with the H2/D2 moiety positioned in minima at either end of the I2 molecule, along the bond axis. Those features associated with complexes containing para-H2(j = 0), ortho-H2(j = 1), orthoD2(j = 0), and para-D2(j = 1) are also assigned. The linear conformers are found to be more strongly bound than the T-shaped conformers with binding energies of 118.9(1.9) cm1 versus 91.393.3 cm1 for the ortho-H2 3 3 3 I2 complexes and 144.2(2.1) cm1 versus 107.9 cm1 for the para-D2 3 3 3 I2 complexes, respectively. Electronic structure calculations of the complexes containing ICl and I2 with H2, He, Ne, and Ar were performed to reveal the nature of the interactions and to shed insight into the origins of the different binding energies. The most stable minima in the H2/D2 þ I2(B,υ0 ) excited-state potentials have T-shaped geometries. Calculated energies and probability amplitudes of the excited-state levels provide insight into the different excited-state intermolecular vibrational levels accessed by transitions of the two ground-state conformers.
1. INTRODUCTION AND BACKGROUND Spectroscopic investigations of van der Waals complexes have proven to be particularly productive in terms of allowing researchers to gain insights into long-range intermolecular forces and an array of energy-transfer mechanisms. The three-atom, rare gasdihalogen van der Waals complexes (Rg 3 3 3 XY) have become benchmark systems for such studies and have continued to receive significant experimental and theoretical scrutiny over the past 30 years.13 It is now accepted that the Rg þ XY groundstate potential energy surfaces (PESs) have multiple minima; one minimum is in the T-shaped orientation, where the Rg atom is localized in the toroidal potential well positioned orthogonal to and around the X—Y bond axis, and there is a second minimum at each end of the dihalogen referred to as the linear orientation.211 Largely because of corresponding calculations, it is also understood that the transitions originating from the ground-state, T-shaped conformers access the lowest-lying intermolecular vibrational level within the excited electronic state of XY. The transitions originating from the ground-state, linear conformers access two regions of the excited state potential. Excitation to the dissociative regions of the intermolecular potentials gives rise to broad continuum signals. High-lying intermolecular levels r 2011 American Chemical Society
within the excited-state PES are also accessed by excitation of linear conformers.1218 These excited-state levels resemble internal, hindered or free-rotor levels with the Rg atom delocalized about the dihalogen molecule. The increase in size from a three-atom, atomdiatom van der Waals complex to a four-atom, diatomdiatom system not only increases the degrees of freedom within the complex, but it can also change the roles of long-range, electrostatic, induction, and dispersion interactions on the multidimensional PESs.19 To probe the differences between the three-atom and four-atom interactions, we extended the ro-vibronic investigations of the Rg 3 3 3 XY systems to H2 þ ICl and D2 þ ICl.20,21 The H2 3 3 3 ICl and D2 3 3 3 ICl spectra recorded in each ICl BX, υ0 0 vibronic region contain features attributed to transitions of complexes containing para(p)-H2(j = 0) and ortho(o)-H2(j = 1) or o-D2(j = 0) and p-D2(j = 1) as well as two different ground-state conformers Special Issue: J. Peter Toennies Festschrift Received: February 16, 2011 Revised: April 19, 2011 Published: May 13, 2011 7368
dx.doi.org/10.1021/jp201549d | J. Phys. Chem. A 2011, 115, 7368–7377
The Journal of Physical Chemistry A for each.20,21 The linear conformers of the H2/D2 3 3 3 ICl(X,υ00 = 0) complexes are significantly more strongly bound than the T-shaped conformers; the binding energy of the linear o-H2 3 3 3 I35Cl(X,υ00 = 0) complex, for instance, was found to be 186.4(3) cm1,20,21 and the binding energy of the T-shaped conformer was bracketed between 82.8(3)89.6(3) cm1.22 Because of this, we speculated that electrostatic interactions, particularly dipolequadrupole interactions, serve to enhance the binding strength of the linear conformer. As we will show in this work, this difference is reflected in the changes in the electron densities of the monomers upon complexation to form the hydrogendihalogen complexes. As described in this article, we have expanded these earlier studies to investigate the role of the electrostatic interactions in this family of weakly bound van der Waals complexes. Specifically, we report here on experiments and calculations aimed at characterizing the H2/D2 þ I2 interactions. With I2, as opposed to ICl, dipolequadrupole interactions are not present, and a comparison of the binding energies of the linear conformers of the H2/D2 3 3 3 I2(X,υ00 = 0) and H2/D2 3 3 3 ICl(X,υ00 = 0) complexes may shed light on the differences between the dipolar and quadrupolar interactions. Comparisons of the binding energies and the electron densities of the H2/D2 3 3 3 I2(X,υ00 = 0) conformers with those of the Rg 3 3 3 I2(X,υ00 = 0) complexes, with Rg = He, Ne, and Ar, are also made to emphasize the differences in the four-atom interactions. Calculations of the H2 3 3 3 I2(B,υ0 = 20) and D2 3 3 3 I2(B,υ0 = 20) intermolecular energies and wave functions indicate the nature of the levels that are accessed in the observed spectra. The calculations indicate that there are a large number of bound intermolecular vibrational levels, of which only a subset are observed experimentally.
2. EXPERIMENTAL METHODS The experimental procedures used have been described in detail previously for H2/D2 3 3 3 ICl20,21 and are only briefly summarized here. Complexes of H2/D2 and I2 were stabilized by passing ≈7.9 bar of a mixture containing between 4 and 10% H2/ D2 in He over a room temperature vessel containing molecular iodine. Most of the experiments were performed using n-H2 or n-D2 in He, although some spectra were recorded with predominantly p-H2 or o-D2 in He to identify those features associated with transitions of the different nuclear spin states of hydrogen. Note that in these latter experiments a fraction of the p-H2(j = 0),
