Rouslan V. Olkhov

Infrared photodissociation spectra of the C–H stretch vibrations of C6H6+–Ar,C6H6+–N2, and C6H6+–(CH4)1–4

Vibrational infrared photodissociation spectra of mass selected C6H6+–Ar, C6H6+–N2, and C6H6+–(CH4)1–4 ionic complexes are recorded in the spectral range of the C–H stretching vibrations. Transitions at 3095±15 cm−1 occur in all spectra and are assigned to C–H stretch fundamentals of the benzene cation in its 2E1g electronic ground state. In the case of the C6H6+–(CH4)1–4 complexes, additional transitions at 2904±7 and 3010±24 cm−1 are observed and attributed to the symmetric and antisymmetric C–H stretch vibrations of the CH4 ligands, ν1 and ν3. The deduced C–H stretching vibrations of C6H6+ in the 2E1g ground state are roughly 30 cm−1 higher than the corresponding frequencies in the 1A1g electronic ground state of the neutral species, indicating that the C–H bonds become stronger upon removal of an electron from the highest occupied e1g orbital of C6H6.

Infrared photodissociation spectra of CH3+–Arn complexes (n=1–8)

Infrared photodissociation spectra of the ionic complexes CH3+–Arn (n=1–8) have been recorded in the vicinity of the ν3 asymmetric stretching vibration of the CH3+ monomer. The CH3+–Ar dimer has also been investigated in the spectral range of the first CH stretching overtones, resulting in the characterization of its 2ν1, ν1+ν3, and 2ν3 vibrational states at the level of rotational resolution. The spectrum of CH3+–Ar is consistent with a pyramidal C3v minimum structure of the complex predicted by ab initio calculations at the MP2 level, whereby the Ar atom is attached to the empty 2pz orbital of the CH3+ moiety. The rotationally resolved ν3 spectrum of the CH3+–Ar2 trimer indicates that the two Ar atoms are located on opposite sides of the CH3+ moiety on the C3 axis, with significantly differing intermolecular C–Ar bond lengths. The splittings observed in the trimer spectrum are attributed to a tunneling motion between two equivalent C3v minimum configurations via a symmetric D3h transition state. The spe...

Spectroscopic and ab initio studies of ionic hydrogen bonds: the O–H stretch vibration of SiOH+–X dimers (X=He, Ne, Ar, N2)

Abstract Infrared photodissociation spectra of SiOH + –X dimers with X=He, Ne, Ar, and N 2 are analyzed in the vicinity of the O–H stretch ( ν 1 ) vibration of SiOH + . The observed complexation-induced red-shifts, Δ ν 1 =−8.4(3.0), −35.7(6), −217.42(1), and −517.1(5) cm −1 , are consistent with the linear proton-bound global minimum structures predicted by ab initio calculations at the MP2 level of theory. Empirical relations between the magnitude of Δ ν 1 and the interaction strength of the ionic hydrogen bond are discussed.

INTERMOLECULAR INTERACTION IN PROTON-BOUND DIMERS. INFRARED PHOTODISSOCIATION SPECTRA OF RG-HOCO+ (RG = HE, NE, AR) COMPLEXES

Abstract Infrared photodissociation spectra of the proton-bound Rg–HOCO + ionic complexes (Rg=He/Ne/Ar) have been recorded in the vicinity of the ν 1 (OH stretch) vibration. The observed complexation-induced red shifts (Δ ν 1 ) are 47, 120, and 704 cm −1 for the He, Ne and Ar containing species and correlate linearly with the proton affinity of the rare-gas atom. Weak transitions in the 100–220 cm −1 range to the blue of the ν 1 fundamentals are assigned to combination bands of ν 1 with the intermolecular stretching mode, ν s . The geometries, vibrational frequencies and interaction energies of the Rg–HOCO + complexes have been investigated by ab initio calculations at the MP2 level, and the results are in good agreement with the experimental data.

INFRARED PHOTODISSOCIATION SPECTRA OF ISOMERIC SIOH+-ARN (N= 1-10) COMPLEXES

Abstract Infrared photodissociation spectra of mass-selected SiOH + –Ar n complexes ( n =1–10) have been recorded in the vicinity of the OH stretch ( ν 1 ) vibration of SiOH + . The SiOH + –Ar dimer has also been investigated by ab initio calculations at the MP2 level and two stable isomers have been found. The linear proton-bound global minimum ( D e =1117 cm −1 ) and the less stable T-shaped silicon-bound geometry ( D e =938 cm −1 ) are separated by an isomerization barrier of ∼500 cm −1 . Three rotationally resolved bands observed in the dimer spectrum are assigned to the ν 1 fundamental (3444.9 cm −1 ) and the ν 1 + ν s (3554.6 cm −1 ) and ν 1 +2 ν b (3581.5 cm −1 ) combination bands of the linear isomer, whereas a weak unresolved band centered at 3673 cm −1 is attributed to the ν 1 fundamental of the T-shaped isomer. The assignments are based on the rotational analysis, vibrational frequencies and comparison with the ab initio results. Systematic ν 1 vibrational band shifts observed in the spectra of larger clusters have allowed to monitor the cluster growth and identify coexisting isomers.

Hindered rotation in ion-neutral molecular complexes: The ν1 vibration of H2–HCO+ and D2–DCO+

Infrared spectra of the mass selected ionic complexes H2–HCO+ and D2–DCO+ have been recorded in the vicinity of their ν1 vibrations (H2/D2 stretch) by means of photofragmentation spectroscopy. The anomalous rotational constants obtained by fitting the observed line positions to a semirigid Watson A-type Hamiltonian reflect the appreciable zero-point excursions of the H2/D2 molecule. Barriers for this internal motion are estimated utilizing a simple atom–diatom hindered rotor Hamiltonian. According to this one-dimensional model, the barrier increases by about 15% upon vibrational excitation which is mainly attributed to electrostatic effects.

Infrared predissociation spectra of HeHO2+ and NeHO2+: prediction of thegn1 frequency of HO2+

Abstract Infrared spectra of theν1 vibration (O H stretch) of the proton-bound He HO2+ and Ne HO2+ ionic complexes have been obtained by means of photodissociation spectroscopy in a tandem mass spectrometer. The measured vibrational frequencies of 2905 and 2761 cm−1 allow for an estimation of the unknownν1 frequency of the HO2+ monomer as3020 ± 40 cm−1.

Intermolecular interaction in the CH3+–He ionic complex revealed by ab initio calculations and infrared photodissociation spectroscopy

The infrared photodissociation spectrum of the degenerate asymmetric CH stretch (ν3) vibration of the CH3+–He ionic complex has been recorded. The rotational structure and vibrational frequency of the observed transition are consistent with a π-bonded C3v cluster geometry where the He ligand is attached to the 2pz orbital of the central C atom of CH3+. The intermolecular bond in the ground vibrational state is characterized by an averaged intermolecular separation of Rcm=2.18 A. The origin of the ν3 vibration of the complex is slightly blue shifted (7 cm−1) compared to the monomer frequency, indicating that vibrational excitation is accompanied by a slight destabilization of the intermolecular bond. Ab initio calculations at the MP2/aug-cc-pVTZ# level of theory confirm that the π-bonded configuration corresponds to the global minimum structure of the complex (De=707 cm−1, Re=1.834 A). The calculated intermolecular potential energy surface of this “disk-and-ball” ionic complex reveals substantial angular-radial couplings in the region of the global minimum, which account for the large discrepancy between vibrationally averaged and calculated equilibrium intermolecular separations, Rcm−Re=0.35 A.The infrared photodissociation spectrum of the degenerate asymmetric CH stretch (ν3) vibration of the CH3+–He ionic complex has been recorded. The rotational structure and vibrational frequency of the observed transition are consistent with a π-bonded C3v cluster geometry where the He ligand is attached to the 2pz orbital of the central C atom of CH3+. The intermolecular bond in the ground vibrational state is characterized by an averaged intermolecular separation of Rcm=2.18 A. The origin of the ν3 vibration of the complex is slightly blue shifted (7 cm−1) compared to the monomer frequency, indicating that vibrational excitation is accompanied by a slight destabilization of the intermolecular bond. Ab initio calculations at the MP2/aug-cc-pVTZ# level of theory confirm that the π-bonded configuration corresponds to the global minimum structure of the complex (De=707 cm−1, Re=1.834 A). The calculated intermolecular potential energy surface of this “disk-and-ball” ionic complex reveals substantial angular-r...

Microsolvation of the methyl cation in neon: Infrared spectra and ab initio calculations of CH3+–Ne and CH3+–Ne2

Rotationally resolved infrared photodissociation spectra of the degenerate asymmetric C–H stretch vibration (ν3) of the CH3+–Ne and CH3+–Ne2 ionic complexes have been recorded. The rotational structure and vibrational frequencies are consistent with π-bound cluster geometries, where the Ne ligands are attached to either side of the 2pz orbital of the central C atom of the methyl cation, leading to C3v and D3h symmetric structures for the dimer and trimer. The intermolecular bonds in the ground vibrational state are characterized by averaged separations of Rc.m.=2.30 A in the dimer and 2.34 A in the trimer. The origins of the ν3 band are blueshifted by 11.5 and 21.5 cm−1 compared to the monomer frequency, indicating that vibrational excitation is accompanied by a small and additive destabilization of the intermolecular bond. Ab initio calculations at the MP2/aug-cc-pVTZ# level confirm that the π-bound configurations correspond to the global minimum structures for both the dimer (De=958.5 cm−1, Re=2.1347 A,...

Intermolecular interaction in an open-shell π-bound cationic complex: IR spectrum and coupled cluster calculations for C2H2+-Ar

The intermolecular potential energy surface (PES) of Ar interacting with the acetylene cation in its (2)Pi(u) ground electronic state is characterized by infrared photodissociation (IRPD) spectroscopy and quantum chemical calculations. In agreement with the theoretical predictions, the rovibrational analysis of the IRPD spectrum of C(2)H(2) (+)-Ar recorded in the vicinity of the antisymmetric CH stretching fundamental (nu(3)) is consistent with a vibrationally averaged T-shaped structure and a ground-state center-of-mass separation of R(c.m.) = 2.86 +/- 0.09 A. The nu(3) band experiences a blueshift of 16.7 cm(-1) upon complexation, indicating that vibrational excitation slightly reduces the interaction strength. The two-dimensional intermolecular PES of C(2)H(2) (+)-Ar, obtained from coupled cluster calculations with a large basis set, features strong angular-radial coupling and supports in addition to a global pi-bound minimum also two shallow side wells with linear H-bound geometries. Bound state rovibrational energy level calculations are carried out for rotational angular momentum J = 0-10 (both parities) employing a discrete variable representation-distributed Gaussian basis method. Effective spectroscopic constants are determined for the vibrational ground state by fitting the calculated rotational energies to the standard Watson A-type Hamiltonian for a slightly asymmetric prolate top.