Structures and spectroscopic properties calculated for C 6 H 7 + and its complexes with Ne, Ar, N 2, or CO 2 Peter Botschwina and Rainer Oswald Institute of Physical Chemistry, University of Göttingen, Tammannstr. 6, Göttingen, Germany References: P. Botschwina and R. Oswald, J. Phys. Chem. A 116, 3448 (2012). P. Botschwina and R. Oswald, J. Chem. Phys. 136, (2012).

Gas-phase spectroscopic information on the electronic ground state of the benzenium ion (C 6 H 7 + ) is still very scarce and essentially limited to IRMPD spectroscopy of two bands at 1228 and 1433 cm -1 (Jones et al., 2003). More data are available through IRPD spectroscopy of complexes C 6 H 7 +  L (L = Ar, N 2,....) by the Dopfer and Duncan groups and through p-H 2 matrix- isolation IR spectroscopy of C 6 H 7 + by Y.-P. Lee and coworkers. Previous spectroscopic work on C 6 H 7 + and C 6 H 7 +  L

Present theoretical work I) Electronic structure calculations by explicitly correlated coupled cluster theory, mostly CCSD(T)-F12x methods as incorporated in MOLPRO (version and higher)  T. B. Adler, G. Knizia, H.-J. Werner, J. Chem. Phys., 2007, 127,  G. Knizia, T. B. Adler, H.-J. Werner, J. Chem. Phys., 2009, 130,  H.-J. Werner, G. Knizia, T. B. Adler, O. Marchetti, Z. Phys. Chem., 2010, 224, 493.  H.-J. Werner, G. Knizia, T. B. Adler, Fr. R. Manby, in: Recent Progress in Coupled Cluster Methods: Theory and Applications, P. Carsky, J. Ritter, J. Paldus (Eds.), Springer  CCSD(T)-F12x methods are very good approximations to standard CCSD(T) while approaching the basis set limit much more quickly.  The basis superposition error (BSSE) is much less important than with standard CCSD(T).

AO basis sets for electronic structure calculations Cation (C 6 H 7 + )Ligands (Ne, Ar, N 2, and CO 2 ) cc-pVnZ-F12 (VnZ-F12) a aug-cc-pVnZ (AVnZ) b (including tight d functions for Ar) Individual combinations (VDZ-F12, AVTZ) or briefly (D, T) (VTZ-F12, AVQZ) or briefly (T, Q) (VQZ-F12, AV5Z) or briefly (Q, 5) a) K. A. Peterson, T. B. Adler, and H.-J. Werner, J. Chem. Phys., 1002, 96, b) T. H. Dunning, Jr. and coworkers.

II)  Harmonic vibrational wavenumbers for free C 6 H 7 + by CCSD(T*)-F12a/VTZ-F12  anharmonic contributions for C 6 H 7 + by VPT2 with B2PLYP-D/VTZ quartic force field*  Harmonic wavenumber shifts for complexes by B2PLYP-D/(VTZ, AVTZ)* *using Gaussian 09 B2PLYP-D: T. Schwabe and S. Grimme, PCCP 9, 3397 (2007).

Recommended equilibrium structure for C 6 H 7 + from CCSD(T*)-F12a/VTZ-F12 + corrections Recommended ground-state rotational constants A 0 = 5442 MHz, B 0 = 5311 MHz, C 0 = 2731 MHz Equilibrium dipole moment:  e = D

VibrationCCSD(T*)-F12a+ B2PLYP-Dp-H 2 a C 6 H 7 +  Ar b  4 (a 1 ) (a 1 ) (a 1 ) (a 1 ) (a 1 ) (a 1 ) (b 1 ) d 18 (b 1 ) (b 1 ) (b 1 ) (b 2 ) (b 2 ) (b 2 ) (b 2 ) d 32 (b 2 ) c (b 2 ) a Bahou et al. (2012). b Douberly et al. (2008). c Probably too low. d Questionable.

Solcà and Dopfer (2003) Douberly et al. (2008) C 6 H 7 + · Ar C 6 H 7 + · N 2 Infrared photodissociation (IRPD) spectra

C 6 H 7 + and C 6 H 7 + · Ar: aromatic stretching vibrations BandCCSD(T*)-F12a+B2PLYP-D a C 6 H 7 + · Ar b harmonicanh. contr.IRPD 1 (a 1 ) (2) 2 (a 1 ) (6) (a 1 ) (0) 24 (b 2 ) (8) (b 2 ) (3) a IR intensities (DHA, in km mol -1 ) in parentheses. b Douberly et al. (2008). Bands observed at 3006 and 3035 cm -1 are too low to be assignable to aromatic stretching vibrations; and might be suitable candidates.

B3LYP G(d,p) Douberly et al. (2008) 211 cm -1 (CP corr.) 179 cm -1 (CP corr.) MP2/6-311G(2df, 2pd) Solcà and Dopfer (2003) Equilibrium structures and D e values from previous work.

Definition of intermolecular coordinates for C 6 H 7 +  CO 2 (an analogous figure applies for L = N 2 ). (a) Path I perpendicular to ring-plane; (b) Path II: in ring-plane.

Radial energy profilesPE curves for in-plane argon approach C 6 H 7 + · Ar (CCSD(T*)-F12a/(T,Q)) no hydrogen bonds to aromatic nor to methylenic hydrogens

Energetically most favourable structures. D e in cm -1. C 6 H 7 +  NeC 6 H 7 +  Ar

C 6 H 7 + · N 2 CCSD(T*)-F12a/(T, Q) C 6 H 7 + · CO 2 radial energy profiles Perp M1 Perp M2 Ipl M1 Ipl M2 Ipl M3

Energetically most favourable structures for C 6 H 7 + · L (L = N 2 and CO 2 ) Equilibrium dissociation energies (D e in cm -1 ) from CCSD(T)-F12x calculations with large (Q, 5) basis set (1014 and 1141 cGTOs), including non-rigidity effects.

Perp M2: hydrogen-bonded structure with D e  785 cm -1 and barrier height of ca. 80 cm -1 for migration Perp M2  Perp M1.

C 6 H 7 + · L complexes: Summary For L = Ne, Ar, and N 2, the largest D e value is obtained for the "  -bound" structure Perp M1. For L = CO 2, the lowest energy minimum corresponds to structure Ipl M1 (CO 2 in ring-plane and adjacent to CH 2 group). Largest change occurs for CH 2 scissoring vibration ( 7 ): -21 cm -1 and intensity enhancement by 29 km mol -1 (26 %). D 0 estimates for Ipl M1 and Perp M1 are 1372  50 and 1330  50 cm -1. There is no indication of hydrogen bonds to aromatic H atoms. Instead, local in-plane minima are found for structures with the ligand pointing to the centre of one of the six CC bonds.