Mark J. Watkins

Evidence for a strong intermolecular bond in the phenol⋅N2 cation

The phenol⋅N2 complex cation has been studied with a combination of two-color resonant zero kinetic energy (ZEKE) and mass analyzed threshold ionization (MATI) spectroscopies to probe the interaction of a polar cation with a quadrupolar solvent molecule. Extended vibrational progressions are observed in three modes which are assigned as the in-plane bend (35 cm−1), the stretch (117 cm−1), and in-plane wag (130 cm−1) intermolecular vibrations, and are consistent with a structure where the N2 forms a directional bond to the phenol OH group in the plane of the aromatic ring. Ab initio calculations at the UMP2/6-31G*, UHF/cc-pVDZ, and UMP2/cc-pVDZ levels of theory support this assignment. The spectra also provide a value for the adiabatic ionization energy (67 423 cm−1±4.5 cm−1) and an estimate of the dissociation energy of the cluster (1650±20 cm−1) which illustrate that the quadrupolar nitrogen molecule binds considerably more strongly to the phenol cation than a rare gas atom. These results constitute the ...

Adiabatic and vertical ionization energies of 1,4-diazabicyclo[2,2,2]-octane measured by zero electron kinetic energy spectroscopy and Rydberg extrapolation

We have used a combination of high-n Rydberg spectroscopy and zero electron kinetic energy (ZEKE) spectroscopy to measure the adiabatic and vertical ionization energies of 1,4-diazabicyclo[2,2,2]-octane to high precision. A comparison of the two methods revealed that where extended and unperturbed Rydberg series can be observed, the precision with which the ionization energy of a molecule can be measured is up to six times better via Rydberg extrapolation than with high resolution ZEKE spectroscopy. The difference in precision derives, not from any inherent shortcoming in the resolution achievable with ZEKE spectroscopy, but from errors that arise in evaluating the field ionization redshift. Accurate vertical ionization energies of 59 049.0±0.1 and 58 889.9±0.1 cm−1 were obtained for ionization to the 24+1 and 25+1 vibrational levels in the ion. A value for the adiabatic ionization energy of 58 033.3±0.3 cm−1 has been measured by ZEKE spectroscopy for the first time. The precision with which we have been ...

Electronic spectroscopy of the Au(6p)–Kr complex

We report electronic absorption spectra, recorded using one- and two-color resonance-enhanced multiphoton ionization spectroscopy, of the Au-Kr complex. The transition is localized on the gold atom, and corresponds to a 6p<--6s atomic excitation; we observe transitions to the D (2)Pi(1/2) and D (2)Pi(3/2) spin-orbit states. In addition, we report the results of ab initio calculations, which consider electronic states arising from the 6 (2)S, 5 (2)D, and 6 (2)P atomic energy levels of Au. Further, we also report an accurate value for the dissociation energy of the ground state of Au-Kr, based on basis set extrapolated RCCSD(T) calculations. The experimental results are discussed in the light of the theoretical ones.

An examination of structural characteristics of phenylacetylene by vibronic and rovibronic simulations of ab initio data

The structural properties of phenylacetylene have been investigated in the S(0)((1)A(1)) neutral ground and S(1)((1)B(2)) and S(2)((1)A(1)) singlet excited states and the D(0)((2)B(1)) cationic state using both rovibronic and multidimensional Franck-Condon simulations from data determined via correlated ab initio methods. Results are compared to experimental and ab initio data reported in the literature. (10,10)-CASSCF and a hybrid CASSCF/SACCI frequency analysis using the cc-pVDZ Dunning basis set have been employed to produce vibronic simulations of REMPI/FES, dispersed fluorescence, TPES and MATI spectra. Calculated rotational constants are used where appropriate to compare to rotationally resolved experimental studies. Whilst the simulations are of generally good quality, it is apparent that the distortion of the ring along the long axis upon electronic excitation is underestimated, resulting in smaller predicted changes in ipso and para CCC bond angles and weaker activities in the 6a and 9a modes compared with experiment. Simulations of one-photon MATI spectra on the other hand, which do not rely on excited state methodologies, compare very well with experiment, suggesting that the neutral and cationic ground state geometries are quite accurate, as are the predicted changes in geometry accompanying ionisation. Simulated rotational and vibrational profiles, as well as other calculated physical data, show good agreement with the numerous experimental and computational studies of phenylacetylene in the literature.

Theoretical study of the X Σ2+ states of the neutral CM–RG complexes (CM=coinage metal, Cu, Ag, and Au and RG=rare gas, He–Rn)

We present high level ab initio potential energy curves for the X Σ2+ electronic states of the CM–RG complexes; where CM is a coinage metal, CM=Cu, Ag and Au and RG is a rare gas, RG=He–Rn. These potentials are calculated over a range of internuclear separations, R, and the energy at each point is corrected for basis set superposition error and extrapolated to the basis set limit. Spectroscopic constants are determined from the potentials so obtained and are compared to available experimental data. The impact of core-valence correlation to the overall interactions within the complexes involving the lighter RG atoms is also considered. We find that there is a surprising continuous decrease in Re in these species from CM-He to CM-Rn and show that this is likely due to a combination of sp hybridization and small amounts of charge transfer.

An experimental and ab initio investigation of the low-frequency vibrations of coumaran

Coumaran (2,3-dihydrobenzofuran) has been studied using a combination of (1+1′) resonantly enhanced multiphoton ionization (REMPI) and zero electron kinetic energy (ZEKE) studies, supported by ab initio molecular orbital calculations, in order to characterize the low wave number vibrational structure of the S1 neutral excited and D0 ionic ground states. These studies focus primarily on the modifying effects of electronic excitation and ionization on the balance of forces driving the S1 and D0 equilibrium structures toward or away from planarity. The results suggest that coumaran retains a puckered structure in the S1 state, having a barrier significantly smaller than that in the electronic ground state, but is apparently pseudo-planar or weakly puckered in the cation ground state. In each state the drive towards or away from planarity results from a competition between decreasing bond order in the aromatic system which increases torsional interactions thereby favoring a higher barrier and an increase in b...

A complete active space self-consistent field (CASSCF) ab initio study of phenol–N2: the properties of a weak hydrogen-bonded system in its S1 excited state

The hydroxy-bound isomer of the phenol–N2 van der Waals complex has been studied, together with the phenol monomer, at the CAS(8,7)/6-31G* and CAS(8,7)/cc-pVDZ levels of theory in the S1 electronically excited state as well as the neutral and cation ground states. Calculated geometries, excitation energies, harmonic vibrational frequencies and rotational constants were in good agreement with experiment, especially when using the Dunning cc-pVDZ basis set, although the greatest deviation from observed values predictably occurred for the S1 excited state. In particular, where rotational constants calculated for phenol were reproduced to within a few megahertz of experiment for the S0 and S1 states, for phenol–N2, rotational fits to the REMPI spectrum showed the CASSCF calculations to have underestimated rotational constants, particularly the A″ and A′ values, by about 7%. Binding energies for the complex compare very well to experimental values for S0 and D0 but were underestimated by about 20% for the S1 state. Vibrational analyses of inter- and intramolecular modes showed good agreement with literature for the neutral and ionic ground states of phenol–N2. However, for the S1 excited state, although good agreement was achieved for the a′ symmetry modes, those of a″ symmetry showed significant errors. The results obtained for the electronically excited state demonstrate that calculations at this level provide a good description of the S1 state and can genuinely be a useful aid to assignment of REMPI/LIF excitation spectra of van der Waals complexes of intermediate bond strength.

The effect of weak interactions on the ring puckering potential in the coumaran–argon van der Waals complex: Experimental and ab initio analysis of the intermolecular and low-frequency intramolecular vibrations

The coumaran–argon van der Waals (vdW) complex has been investigated using a combination of (1+1′) REMPI and ZEKE spectroscopy, supported by ab initio molecular orbital calculations. Coumaran (2,3-dihydrobenzofuran) has a puckered, nonplanar equilibrium structure in the electronic ground state which allows for the formation of two energetically nonequivalent π-bound geometrical conformations. The experimental observation of bands attributable to two isomers in the REMPI spectrum is consistent with a significant barrier to planarity existing in both S0 and S1 states. The two isomers are related through the ring-puckering motion but the interaction of the argon atom with the monomer results in an induced asymmetry in the potential which transforms the ring-puckering tunnelling motion in the isolated monomer to a localized vibration near nonequivalent local minima. Both REMPI and ZEKE spectra show rich vibrational structure, characteristic of excitation of the long axis van der Waals bending mode and the str...

Resonance enhanced multiphoton ionization and zero electron kinetic energy spectroscopy of the DABCO–N2 van der Waals complex: Divergent energy level spacings as evidence for an offset parallel structure

The DABCO–N2 van der Waals complex has been investigated using a combination of (1+1′) resonance enhanced multiphoton ionization and zero electron kinetic energy spectroscopy, supported by ab initio molecular orbital calculations. The observation of extended vibrational progressions of low wave number with diverging vibrational spacings supports an assignment to an offset parallel structure analogous to the 45° canted parallel structures proposed for the nitrogen dimer. The active vibrational mode is assigned to a mixed van der Waals stretch/rocking mode in which the nitrogen solvent undergoes a hindered rotational motion against the DABCO framework in the plane containing the C3 axis in DABCO and the intermolecular axis in N2. The results of counterpoise corrected ab initio calculations support this assignment to the extent that they suggest that a parallel structure is the most stable with a cross structure identified as a transition state. No experimental evidence is found for the existence of other st...