Iñigo Unamuno

Isomer structures and vibrational assignment of the methyl-p-aminobenzoate(H2O)1 complex

Methyl-p-aminobenzoate(H2O)1 complex, henceforth MAB(H2O)1, prepared by pulsed supersonic expansion, has been examined by a broad range of laser based spectroscopic, mass and isomer selective techniques and density functional theory (DFT) calculations, in order to identify its isomer structures, ionization energies, and vibrational frequencies. The experimental techniques used include laser induced fluorescence (LIF), mass resolved excitation spectroscopy (MRES) either with one (REMPI) or two laser colors (R2PI), laser excited dispersed emission (DE), high resolution MRES, pressure controlled R2PI, hole burning (HB) spectroscopy, and photoion fragmentation threshold (PIFT). Experimental results have been interpreted, rationalized and extended with density functional theory (DFT) computations at the B3LYP/6-31G and B3LYP/6-31+G* levels. Although bare MAB molecule have four possible solvation sites, prone to yielding hydrogen bonds with the water molecule, LIF, R2PI, and HB spectroscopy of MAB(H2O)1 only pi...

Experimental and theoretical study of the structures and binding energies of eugenol (H2O)n,n=0–2

Eugenol (4-Allyl-2-methoxyphenol), a phenol-derivative with an intramolecular -OH ...OCH(3) hydrogen bond (H bond), has been studied in a supersonic expansion using a number of complementary laser spectroscopic techniques. The mass-resolved excitation spectrum of eugenol and its water complexes are reported for the first time. The most intense set of bands on the resonantly enhanced multiphoton ionization (REMPI) spectrum of eugenol originate in a conformer whose S(1)<--S(0) transition is at 35 202 cm(-1) and the ionization threshold at (I(0)<--S(0)) 62 544+/-150 cm(-1) (7.755+/-0.019 eV). In addition, two low intensity features redshifted with respect to the 0(0) (0) transition have been identified as due to a second, less stable conformer. Ab initio calculations show that the potential energy landscape depicts at least three minima associated with one folded and two extended conformers, one of which is the most stable. Clusters of eugenol/water were prepared in a supersonic expansion by seeding eugenol and water in noble gas He and examined by two-color REMPI (R2PI) and IR-UV double resonance spectroscopies. Only one single isomer was observed for both 1:1 and 1:2 complexes, in contrast with the several stable conformers provided by the computations. The dissociation energies of the 1:1 and 1:2 complexes have been determined by the fragmentation threshold method and the results compared with those from ab initio calculations conducted at the B3LYP and MP2 levels with a variety of basis sets.

GROUND AND FIRST ELECTRONIC EXCITED STATE VIBRATIONAL MODES OF THE METHYL-P-AMINOBENZOATE MOLECULE

Abstract Laser induced fluorescence, dispersed emission and two-colour ionisation spectroscopy (including the ionisation energy threshold) are suggested as an appropriate set of experimental methods for drawing the ground and excited state vibrational spectra of large organic molecules in presence of solvated molecules and dimers. In this Letter, they have been applied to obtain the vibrational spectrum of methyl- p -aminobenzoate (MAB). Parallel ground state ab initio calculations have enabled the geometry to be determined and the vibrational modes to be accurately assigned. The comparison of the ground and the first electronic excited state vibrational band wavenumbers shows an excellent agreement except for the excited state bands observed at 52.2, 565.6 and 579.7 cm −1 .

Experimental determination of phenol (CH3F)1 complex binding energies in the S0, S1, and I0 states and comparison with ab initio calculations

Weakly bound complexes of phenol (Ph) and fluoromethane (CH3F) formed in a supersonic expansion have been identified by one- and two-color mass-resolved and hole burning spectroscopies. Only one isomer has been observed for the 1:1 complex. Threshold fragmentation has been employed to determine the binding energies of the complex in its ground, S0, and first electronic, S1, states, as well as in the ion ground state, I0, yielding the following results: D0(S0)=1540±50 cm−1, D0(S1)=1713±50 cm−1, and D0(I0)=3932±50 cm−1, respectively. In a complementary study, calculations on the complex geometries and binding energies were conducted at the B3LYP/6-31+G* and the MP2/6-31+G* levels. It has been shown that the binding energies computed at the MP2/6-31+G* level are in excellent agreement with the experimental values, whilst those calculated at the B3LYP/6-31+G* level underestimate them by nearly 30%, probably due to the poor description of the dispersion forces.

Structure and ground and first electronic excited state vibrational modes of the ethyl-p-aminobenzoate conformers

Abstract A combined theoretical and experimental study of ethyl- p -aminobenzoate (EAB) molecule to substantiate the number of conformers, identify their electronic 0 0 0 transitions, vibrational bands, structures and ionisation energies is reported. The experimental study is mainly based on pump–probe laser techniques on the sample molecule prepared in a supersonic expansion beam and includes one-colour and two-colour resonance enhanced multiphoton ionisation (REMPI, R2PI) hole burning and dispersed emission spectroscopies. The spectra are rationalised with the help of density functional theory (DFT) computations at the B3LYP/6-31+G* level. EAB R2PI spectrum is reported for the first time and the trans and gauche conformers ionisation energy thresholds have been measured to be 7.828 and 7.826 eV, respectively. The small energy difference of the trans and gauche 0 0 0 transitions is suggested to be originated in the different contribution of the ethyl group orbitals to the conformer highest and lowest occupied molecular orbital states. Ground and first electronic excited states vibrational spectra of both conformers have been assigned with the help of the computed DFT vibrational wave numbers. The most intense bands of the vibrational spectra are identified as belonging to ring vibrations: 10a, 17b, 1+skeletal vibration, 5, 17a, 18b among others. Most vibrational modes in both conformers appear at close wave numbers and only the low energy modes, in the region below 400 cm −1 , should be regarded as the characteristic fingerprint of each conformer. No long vibrational progressions have been observed.

Binding energy and structure of the ground, first electronic and ion states of p-methoxyphenethylamine(H2O)1 isomers: a combined experimental and theoretical study

Abstract An extensive laser spectroscopic, structural and computational investigation on the neurotransmitter analogue p -methoxyphenethylamine (MPEA) chromophore weakly bound to one molecule of water, (MPEA(H 2 O) 1 ) is reported. The complex was prepared by supersonic expansion of a gas mixture of seeded MPEA and water molecules in rare gas He. Two well-characterised experimental isomers have been observed, with geometries resulting from large contributions of the ordinary hydrogen bond and other intermolecular forces. Four key properties of the complex and related species are addressed in this paper: the characterisation of the isolated isomers observed spectra, the measurements of the binding energies of the ground, first electronic and ground ion states of the complex, the identification of each isomer spectrum with the computed geometries and the relationship of the complex stable isomers with the precursor conformers. A number of mass resolved laser spectroscopies have been used in order to get a complete set of experimental results, that complemented with ab initio density functional computations at four basis set, finally permitted us to settle and assign the mentioned properties of the two observed MPEA(H 2 O) 1 isomers.

A theoretical and experimental study of the ethyl-p-aminobenzoate (H2O)n (n=1–4) complexes

Weakly bound hydrogen bonded ethyl-p-aminobenzoate/water complexes, referred to henceforth both as EAB/(H2O)n (n=1–4) or by their stoichiometry 1:n, have been investigated with a combined approach of mass and light detector laser spectroscopic techniques and ab initio calculations. The experimental studies follow explorations with laser induced fluorescence (LIF), and include one-color resonant enhanced multiphoton ionization (REMPI), two-color REMPI (R2PI), pressure dependent R2PI and hole burning (HB) spectroscopies. Calculations were conducted at the B3LYP/6-31+G* level and for the 1:1 complex led to the existence of six stable isomers, identified as the experimental origin bands at +4, +6, +13, +89, +96, and +108 cm−1 above the bare EAB 000 transition. It has been shown that three of these bands originate in the EAB trans conformer, while the other three derive from the EAB gauche conformer. None of the experimental methods used lead us to observe the EAB(H2O)2 complex spectrum and the inspection of t...

Laser mass-resolved spectroscopy and theoretical study of methyl-p-aminobenzoate(H2O)n (n=2,3,4) complexes

A combined computational and experimental study of the methyl-p-aminobenzoate(H2O)n, (n=2,3,4) complexes [MAB(H2O)n] is reported. Complexes potential energy surfaces were explored by ab initio density functional theory (DFT) methods, at the B3LYP/6-31G level, and the stable isomer structures and vibrational modes further computed at the B3LYP/6-31+G* level. A set of self-contained experimental techniques, including laser induced fluorescence (LIF), resonance enhanced multiphoton ionization mass-resolved spectroscopy (REMPI), two-color resonance enhanced multiphoton ionization mass-resolved spectroscopy (R2PI), “hole burning” spectroscopy (HB), and two-color ionization thresholds were used to study the spectra and other physical features of the complexes. Of the three title complexes only MAB(H2O)4 has been observed with our experimental methods, while the MAB(H2O)3 was formed by evaporation and MAB(H2O)2 was not detected at all. It has been shown that the observed MAB(H2O)4 complex has only one isomer wit...

Experimental and theoretical study of methyl-p-aminobenzoate/ammonia complexes. I. MAB(NH3)1

A complementary laser spectroscopy and computational study of the MAB(NH3)2–4 complexes, hereafter referred to by its stoichiometry, i.e., 1:2, 1:3, and 1:4, prepared in a supersonic expansion, is reported. Experimental evidence shows the existence of abundant fragmentation cascades, the most notorious being the observation of the 1:4 complex spectrum in the 1:3 and to 1:2 mass channels, in fact, the observed spectra of the 1:2 and 1:3 complexes are not genuine but a consequence of fragmentation. The observed 1:4 complex resonance enhanced multiphoton ionization (REMPI) spectrum has a significant redshift of −1160 cm−1 from the bare MAB 000 transition and appears over a noisy background that decreases, although it does not disappear, in resonance enhanced two-color photo ionization (R2PI) studies. “Hole burning” spectroscopy corroborates the presence of only one 1:4 isomer. Calculations at the B3LYP/6−31+G* level conduct to a number of 1:2, 1:3, and 1:4 stable isomer structures, the most stable being the ...

Structure and identification of the amino-p-phenethylamine conformers

This work reports the identification of the gas phase amino-p-phenethylamine (APEA) conformers with computed DFT stable structures. Two-colour mass-resolved multiphoton ionisation and hole burning spectroscopies have been used to find the number of conformers and record their spectra. The spectra show the presence of four well-characterized conformers, with origin bands paired off as gauche and anti conformers, in clear contrast to the five stable structures predicted by computational methods. Several criteria have been employed to relate the experimental spectra of the conformers to stable computed structures, the ionisation energies being the most reliable for unambiguous identification. The method can be applied to other small- and medium-size biological and organic molecules.