Doris Roth

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 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 OH+–He and OH+–Ne open-shell ionic complexes: Infrared predissociation spectra of the ν1 and ν1+νb vibrations

Midinfrared spectra of the OH+–He/Ne open-shell ionic complexes have been recorded by photofragmentation spectroscopy in a tandem mass spectrometer. The ν1 vibration (OH stretch) and its combination band with the intermolecular bending vibration (ν1+νb) have been observed for both complexes at the level of rotational resolution. The analysis of the spectra shows that both complexes have a linear proton-bound structure in their 3Σ− electronic ground states, with intermolecular center-of-mass separations of 2.60 and 2.65 A for OH+–He and OH+–Ne, respectively. The ν1 vibrational origins are redshifted by 66.3 and 169.9 cm−1 with respect to the corresponding monomer transition indicating that the intermolecular interaction increases upon ν1 excitation. The fine structure of the ν1 (3Σ−←3Σ−) and ν1+νb (3Π←3Σ−) transitions arising from electron spin and vibrational angular momentum of νb have been analyzed in terms of a semirigid Hamiltonian including spin–spin, spin–rotation, and l-type doubling interaction te...

Site-specific interaction between hydrocarbon cations and inert ligands: IR spectra of isomeric C3H3+–L dimers (L = Ne,Ar,O2,N2,CO2)

Infrared (IR) photodissociation spectra of isomeric C3H3+–L dimers with the inert ligands L=Ne, Ar, O2, N2, and CO2 are recorded in the spectral range of the C–H stretch fundamentals. At least two C3H3+ isomers are generated by electron impact ionization of allene (H2CCCH2) in the cluster ion source, namely the cyclopropenyl (c-C3H3+) and propargyl (H2CCCH+) cations. Both C3H3+ ions form weakly-bound adducts with all ligands considered and the attraction in these ion–ligand complexes is dominated by induction and electrostatic forces. In general, the intermolecular interaction in both c-C3H3+–L and H2CCCH+–L increases in the order Ne<Ar<O2<N2<CO2, in line with the polarizabilities, quadrupole moments, and proton affinities of L. Rovibrational analysis of the ν4 vibrations of c-C3H3+–L involving molecular ligands reveals several competing structural binding motifs, including the formation of H-bonds (L=N2, CO2, O2), C-bonds (L=CO2), and π-bonds (L=O2). The preferred binding site and details of the equilibrium structure strongly depend on L. In contrast, the observed H2CCCH+–L dimers feature all H-bonds between L and the acetylenic proton. Ab initio calculations for c-C3H3+–L and H2CCCH+–L at the MP2(full)/6-311G(2df,2pd) level provide additional information about the intermolecular potential energy surface of these ion–ligand complexes and confirm the interpretation of the experimental IR spectra. Photofragmentation branching ratios observed for c-C3H3+–(CO2)n with n=1–3 allow the dissociation energy of the intermolecular H-bonds and C-bonds to be estimated as D0(H/C)=1300±300 cm−1.

Microsolvation of the water cation in neon: Infrared spectra and potential energy surface of the H2O+–Ne open-shell ionic complex

The intermolecular potential of the H2O+–Ne open-shell ionic dimer in its doublet electronic ground state has been investigated by infrared spectroscopy in the vicinity of the O–H stretch vibrations (ν1 and ν3) and ab initio calculations at the unrestricted Moller–Plesset second-order (MP2) level with a basis set of aug-cc-pVTZ quality. The rovibrational structure of the photodissociation spectrum is consistent with a proton-bound planar H–O–H–Ne structure and a Ne–H separation of R0=1.815(5) A. The complexation-induced redshifts are Δν1=−69 cm−1 and Δν3=−6 cm−1, respectively. Tunneling splittings observed in the perpendicular component of the ν3 hybrid band of H2O+–Ne are attributed to hindered internal rotation between the two equivalent proton-bound equilibrium structures. The interpretation of the H2O+–Ne spectrum is supported by the spectrum of the monodeuterated species, for which both the proton-bound and the deuteron-bound isomers are observed (DOH+–Ne, HOD+–Ne). The equilibrium structure of the c...

Intermolecular potential energy surface of the proton-bound H2O+–He dimer: Ab initio calculations and IR spectra of the O–H stretch vibrations

Rotationally resolved infrared photodissociation spectra of the O–H stretch fundamentals (ν1 and ν3) of the H2O+–He open shell ionic complex have been recorded in the doublet electronic ground state. The analysis of the complexation-induced frequency shifts (Δν1 = − 15 cm−1, Δν3 = − 5.1 cm−1) and the rotational structure is consistent with a planar, translinear proton-bound H–O–H–He equilibrium geometry. The derived intermolecular H–He separation and O–H–He bond angle are R0 = 1.756(4) A and φ0 = 175(5)° in the ground vibrational state. The experimental results are in good agreement with the ab initio calculations performed at the unrestricted MP2 level using a basis set of aug-cc-pVTZ quality. The global minimum on the calculated potential energy surface features a slightly translinear proton-bound equilibrium geometry, with an intermolecular separation of Re = 1.6990 A, a bond angle of φe = 173.2°, dissociation energies of De = 425.9 cm−1 and D0 = 158.6 cm−1, and frequency shifts of Δν1 = − 34.4 cm−1 and Δν3 = − 11.6 cm−1. Tunneling splittings in the perpendicular component of the ν3 band are attributed to hindered internal rotation exchanging the two equivalent proton-bound structures ia a planar transition state with C2v symmetry. The calculated barrier for this internal motion amounts to Vb = 202.9 cm−1 and the dimer is closer to the semirigid limit than to free internal rotation. The interpretation of the H2O+–He spectrum is supported by the corresponding spectrum of the monodeuterated HOD+–He complex.

Infrared spectrum and ab initio calculations of the He–HNH+ open-shell ionic complex

Abstract The infrared photodissociation spectrum of the asymmetric N–H stretch vibration ( ν 3 ) of the He–HNH + open-shell ionic complex has been recorded. The complexation-induced frequency red shift, Δ ν 3 =7.0 cm −1 , and the rotational structure of the transition are consistent with a quasi-linear geometry of the complex in the electronic ground state, with a vibrationally averaged intermolecular bond length of 1.90 A. Ab initio calculations performed at the UMP2 level confirm that the quasi-linearity and diradical character of the nitrenium ion in its 3 Σ − g electronic ground state are not changed upon He complexation. The calculated properties of the intermolecular bond ( D e =273 cm −1 , R e =1.82 A) and the frequency shift Δ ν 3 =13.5 cm −1 are in good agreement with experiment.

Infrared spectrum and ab initio calculations of the HNH+–Ne open-shell ionic dimer

The IR photodissociation spectrum of the HNH+–Ne open-shell ionic complex has been recorded in the vicinity of the N–H stretch fundamentals of HNH+ (ν1 and ν3). The rovibrational analysis of the spectrum is consistent with a quasilinear proton-bound HNH+–Ne geometry in the 3Σ− electronic ground state and an intermolecular separation RH–Ne∽1.93 A in the ground vibrational state. Ab initio calculations at the UMP2/aug-cc-pVTZ# level confirm that the diradical character and quasilinearity of HNH+ are only a little affected by the weak interaction with Ne. The dissociation energy of the complex is calculated as De = 530 cm−1.

Potential energy surface and infrared spectrum of the Ar–H2Cl+ ionic complex

The infrared photodissociation spectrum of the Ar–H2Cl+ dimer has been recorded in the vicinity of the Cl–H stretch fundamentals of bare H2Cl+. Eleven Q branches of a strong perpendicular transition of a (near) prolate symmetric top are observed. The position and rotational structure of the band are consistent with an assignment to the free Cl–H stretch fundamental of a proton-bound Ar–HClH+ dimer. The global minimum on the intermolecular potential energy surface of Ar–H2Cl+, calculated at the MP2/aug-cc-pVTZ# level of theory, corresponds to the proton-bound structure with an intermolecular separation of Re=1.97 A and a well depth of De=1860 cm−1. The slightly nonlinear ionic hydrogen bond is directional with large barriers (Vb) for internal rotation of H2Cl+ via planar transition states with C2v symmetry: Vb∼750 and 1330 cm−1 for the bridged (Re=3.45 A, De=1107 cm−1) and chlorine-bound (Re=3.38 A, De=531 cm−1) structures. The molecular constants of the observed transition, ν0=2663.1 cm−1 and A=10.35 cm−1...