17772
J. Phys. Chem. 1996, 100, 17772-17779
High-Resolution Rovibrational Absorption Spectrum of CO2-N2O C. Dutton, A. Sazonov, and R. A. Beaudet* Department of Chemistry, UniVersity of Southern California, UniVersity Park, Los Angeles, California 90089-0482 ReceiVed: July 9, 1996; In Final Form: September 3, 1996X
The rovibrational absorption spectrum of the weakly bound cluster CO2-N2O has been observed in the region of the ν3 CO2 asymmetric stretch (∼2350 cm-1). Clusters were formed by supersonic expansion of a mixture of N2O:CO2 in a 4:1 ratio using He carrier gas. Approximately 340 lines have been assigned. Both parallel and perpendicular transitions were observed. The ground state rotational constants A, B, and C are 0.294 924(12), 0.058 004(6), and 0.048 400(6) cm-1, respectively. The quartic centrifugal distortion constants are Dj ) 5.03(17) × 10-7 cm-1, Djk ) -3.92(9) × 10-6 cm-1, Dk ) 1.26(2) × 10-5 cm-1, δj ) 1.61(92) × 10-8 cm-1, and δk ) 1.77(80) × 10-6 cm-1. Ab initio calculations for several CO2-N2O equilibrium structures have been done on the Hartree-Fock self-consistent field level using a 6-311g* basis set. Møller-Plesset MP2 and MP3 calculations were carried out for the two possible slipped parallel configurations of the dimer. The planar slipped parallel geometry with the oxygen atom of N2O nearest the CO2 monomer was found to be the most stable structure. The spectral constants and stabilization energies were compared to those of the CO2 and N2O homodimers. A planar slipped parallel geometry is observed with the O of N2O nearly over the C of CO2. Rcm and θ are 3.4701 Å and 60.1°, respectively, assuming the CO2 and N2O are parallel to each other.
Introduction van der Waals clusters have been the focus of considerable interest over the past few decades.1-3 The homodimers (CO2)2 and (N2O)2 have been extensively studied, but the mixed dimer CO2-N2O has not yet been identified. For years the structure of (CO2)2 underwent much debate.4-18 Early theoretical and experimental evidence independently supported a T-shape rather than a slipped parallel configuration. It was not until high-resolution infrared studies16-18 were performed that the most stable structure was unambiguously determined to be slipped parallel. Mannik and co-workers’4 early low-resolution infrared study at 192 K in a static gas cell suggested that the most stable structure is T-shaped. They assumed a van der Waals distance of 4.1 Å and supported their conclusion with electrostatic quadrupole-quadrupole calculations. Novick’s supersonically cooled molecular beam electric deflection experiment6 suggested the dimer was polar. In nitrogen and argon matrices, Fredin et al.7 suggested that (CO2)2 possessed no center of symmetry. However, there is no reason to believe that the structure in rare gas matrices is necessarily the same as in free jets. In 1974, Koide and Kihara9 used a modified Lennard-Jones intermolecular potential based on second virial coefficients, electric quadrupole moments, and polarizabilities to calculate potential energy surfaces for several orientations of CO2 in (CO2)2. The slipped parallel configuration had the deepest potential well, with the T-shape only 70 cm-1 higher in energy. A new molecular beam deflection experiment performed by Barton et al.12 showed that the dimer was nonpolar and explained that the polarity observed in Novick’s molecular beam electric deflection experiment6 was probably due to larger clusters. Pubanz and co-workers15 reported coherent Raman detection (CARS) spectra of CO2 clusters. The most stable dimer configuration determined from their spectra contained an inversion center and was therefore determined to be nonpolar. X
Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)02058-8 CCC: $12.00
They did not, however, exclude the presence of a small amount of a polar species. Sub-Doppler resolution infrared spectroscopy on the ν1 + ν3 combination mode of CO2 studied by Jucks and co-workers16 confirmed that the structure of (CO2)2 is slipped parallel. Further sub-Doppler infrared absorption spectroscopy studies by Jucks and co-workers17 in the 2.7 µm region of the Fermi diad ν1 + ν3/2ν02 + ν3 unequivocally reveal that the complex is planar with C2h symmetry. Walsh and co-workers18 obtained a rotationally resolved spectrum of supersonically cooled (CO2)2 in the ν3 asymmetric stretching region of CO2 monomer. They observed both a- and b-type transitions and determined that the structure was a slightly asymmetric prolate top as shown in Figure 1a. The band center of the dimer is 1.61 cm-1 blue-shifted from the CO2 monomer (2349.16 cm-1).19 Since CO2 itself has no permanent dipole but has a large quadrupole moment (-14.34 × 10-40 C m2),20 bonding originates predominantly from quadrupole-quadrupole interactions. The N2O dimer also has been extensively studied21-28 and was determined to have a slipped parallel structure similar to (CO2)2 (cf. Figure 1b). Morales and Ewing24 calculated the lifetimes for the predissociation of vibrationally excited (N2O)2 by using a Morse potential. In their study they found that since the permanent dipole moment of N2O (0.166 × 10-18 esu cm) is too small to contribute significantly to the intermolecular potential of the dimer, the quadrupole-quadrupole interaction is largely responsible for bonding in the dimer. They found that the staggered parallel configuration shown in Figure 1b has the greatest binding energy. Huang and Miller25 obtained rotationally resolved infrared spectra for both (14N14N16O)2 and (15N14N16O)2 by exciting the ν1 + ν3 vibrational mode of N2O. They estimated the expected rotational constants of the three possible T-shaped and two slipped parallel configurations. By comparing the Rcm and θcm of the T-shape and slipped parallel structures with those obtained from experiment, they were able to show unambiguously the structure that agreed best with their data was the centrosymmetric slipped parallel structure shown © 1996 American Chemical Society
Spectrum of CO2-N2O
Figure 1. Experimentally determined (CO2)2 and (N2O)2 geometries.17,25 The center of mass distances are given.
Figure 2. Schematic of the experimental apparatus.
in Figure 1b. The structure obtained by Ohshima and coworkers26 by measuring the infrared absorption spectrum of supersonically cooled (N2O)2 in the ν1 region (∼1280 cm-1) of the N2O monomer is consistent with that obtained by Huang and Miller.25 This paper reports the rovibrational spectrum of the hybrid dimer, CO2-N2O, around the ν3 CO2 asymmetric stretch and compares its geometry to that of the CO2 and N2O homodimers. Overlapping parallel and perpendicular bands are observed. The hybrid dimer has a slipped parallel structure with Rcm ) 3.4701 Å and θ ) 60.1° assuming that the two monomer units are parallel to each another. Experiment Figure 2 shows a schematic drawing of the experimental apparatus. An infrared beam was generated using a liquid nitrogen cooled Pb salt diode laser (Laser Photonics). Single modes were selected by passing the beam through a 0.5 m monochromator (Laser Photonics). After exiting the monochromator, two 4% portions of the infrared beam were
J. Phys. Chem., Vol. 100, No. 45, 1996 17773 redirected, one portion into a 10 cm reference cell containing CO2 (
