Chemical Physics 330 (2006) 138–145 www.elsevier.com/locate/chemphys The aniline–water and aniline–methanol complexes in the S1 excited state G. Piani a, M. Pasquini a,b,I.Lo´pez-Toco´n a,c, G. Pietraperzia a,b, M. Becucci a,b,*, E. Castellucci a,b a LENS, via N. Carrara 1, Polo Scientifico Universita` di Firenze, 50019 Sesto Fiorentino (FI), Italy b Dipartimento di Chimica, Universita` di Firenze, Firenze, Italy c Departamento de Quı´mica-Fı´sica, Universidad de Ma´laga, Spain Received 27 March 2006; accepted 2 August 2006 Available online 5 August 2006 Abstract We report an experimental and theoretical study on the properties of the aniline–water and aniline–methanol jet cooled complexes. In both complexes the ligand (H2OorCH3OH) is hydrogen bonded to aniline, the interaction taking place at the lone pair of the nitrogen, in the amino group. The S1 S0 electronic excitation spectrum in both cases exhibits a very broad and weak band, blue shifted with respect to the origin band of aniline by 700 cmÀ1. Ab initio calculations, at different levels of theory with the cc-pvdz electronic basis set, were performed on aniline–water and predict a strong binding energy decrease in the excited state and a large change in geometry, in agreement with experimental results. Ó 2006 Elsevier B.V. All rights reserved. 1. Introduction [3,4]. Recently, different authors have reported on the properties of the anisole–H2O complex [5–7]. The structure Recent studies have reported on the properties of com- of anisole closely resembles that of phenol: the only differ- plex formed by water with organic molecules in gas phase ence is the change of the OH group with the OCH3 group. supersonic expansion [1,2]. In most of the cases, water acts The electronic transition has practically the same character as a base and the complex is stable, both in the ground and for both molecules [8]. Instead, in the case of water com- in the first electronic excited state. In this respect a repre- plex formation the two systems behave quite differently sentative system is the phenol–water complex. In the phe- as water is binding as an acid to anisole. Therefore, due nol–H2O complex, water is bound as a base to the to the decrease of electron density with electronic excitation phenol OH group [1,3,4]. Due to the changes in the elec- in the oxygen atom lone pairs, the origin of the electronic tronic density with the electronic transition, the hydrogen transition for the complex is blue shifted with respect to bond is stronger in the excited state with respect to the the anisole monomer and the hydrogen bond distance ground state. A measure of this change in the interaction increases. A discussion is still open on the nature of the energy is given by the red shift of the electronic transition structural rearrangement of the anisole–water cluster with in the complex with respect to the isolated phenol [4]. Also electronic excitation and on the possible presence of a sec- the hydrogen-bond distance decreases in the excited state ond relevant interaction point between the oxygen atom of water and the aromatic hydrogen atom in ortho position of anisole. Different results were reported for adenine–H O * Corresponding author. Address: Dipartimento di Chimica, Universita` 2 di Firenze, Firenze, Italy. Fax: +39 055 4572 451. [9,10]. The adenine–H2O complex was clearly observed E-mail address: maurizio.becucci@unifi.it (M. Becucci). with high energy excitation (i.e., photons below 200 nm 0301-0104/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.08.004 G. Piani et al. / Chemical Physics 330 (2006) 138–145 139 wavelength or direct ionization by electronic impact). If the interval between 0 °C and 25 °C. Mass spectra were adenine–H2O complex is excited to the S1 excited state, obtained via multi-photon ionization with different photon then the corresponding cation is observed only if a probe energy, in-resonance and out-of-resonance for the aniline– pulse (used for ionization) is provided with a delay not water complex. The same procedures were applied for the longer than 220 fs with respect to the pump pulse. The study of the aniline–methanol complex. structure of the adenine–H2O 1:1 complex has not been determined by experiments but different models for this 2.2. Ab initio methods system have been reported [11,12]. In some of these models, water is connected to a hydrogen atom of the amino group In order to evaluate the properties of the aniline–H2O and one of the aromatic N atoms [11]. complex in the excited state, we have followed the guide- In this respect, it seems quite interesting to investigate lines provided by Fang for a similar study on phenol– the properties of the simplest aromatic amino molecule, H2O [20]. The equilibrium structure and the interaction i.e., aniline and the complex it forms with water. Our aim energy of the aniline–H2O complex have been calculated is to contribute, with experimental data and quantum cal- by second-order Møller–Plesset (MP2) perturbation theory culation, to the description of the changes in the properties and complete active space-self consistent field (CASSCF) of clusters with electronic excitation. methods in the ground electronic state, S0, and by single It has been shown by microwave spectroscopy experi- configuration interaction (CIS) and CASSCF methods in ments [17] that, in the ground state, aniline acts as a base the first excited electronic state, S1. In order to account with water placed in its symmetry plane above the aromatic for the correlation energy, the CASSCF calculations were ring, connected to the amino group via hydrogen bonding, followed by single point CASPT2 energy calculations. the hydrogens of the amino group being on the opposite The cc-pvdz basis set has been employed in all calculations side of water. The properties of aniline in the gas phase as suggested by previous reports [21,22]. We also used an have been studied in detail both in the ground and the first augmented (aug-cc-pvdz) basis set in order to know its excited electronic state by rotationally resolved experi- effect on the results. The CASSCF active space includes ments [13–15]. Very recently, photon-induced dissociation all p electrons and p orbitals of the aromatic ring (three of anionic clusters containing aniline has been reported. p bonds, three corresponding antibonding orbitals) and There, the electron photo-detachment process have been the nitrogen n lone pair of aniline. This results in an active studied for negative ionic clusters formed by iodide with space of eight electrons in seven orbitals and is denoted by aniline [16]. By the measure in coincidence of the detached CAS(8,7). The oxygen lone pair orbitals of water were not electron and the neutral particles, it has been demonstrated included in the active space due to the difference in the that the cluster is dissociating with quantum yield almost orbital energies from those of p orbitals, which are near- unitary after the electron removal. degenerate. In the case of phenol–H2O complex [20],it We report here on the measure, by resonance-enhanced has been shown that the CASSCF convergence is seriously multi-photon ionization spectroscopy (REMPI), of the degraded when the n orbitals of water were included. The S1 S0 electronic transition of aniline–H2O and the clo- interaction energy derived from this type of calculation is sely related aniline–methanol complex, and on quantum subject to the basis set superposition error (BSSE) that calculations aiming at the description of the system in both can be corrected using the counterpoise method of Boys the ground and the excited. and Bernardi [23]. Therefore, the corrected interaction energy is calculated as: 2. Experimental and computational methods AB AB AB Eint ¼ EABðABÞ À EABðAÞ À EABðBÞ Z 2.1. Experimental methods where we define EY(X) as the energy of a molecular sys- tem X at the equilibrium geometry of the molecular system We have made a search for the S1 S0 electronic tran- Y with the set of basis functions related to the molecular sition of the aniline–H2O complex by REMPI spectroscopy system Z. A better evaluation of the binding energy in experiments in both 2 photon-1 colour and 2 photon-2 col- the complex, called stabilization energy, can be obtained our excitation schemes. The laser spectrometer coupled to a introducing the fragment relaxation in the counterpoise time-of-flight mass spectrometer was already described in scheme according to the expression: detail elsewhere [5]. The molecular beam was prepared by E ¼ E þ E ðAÞA þ E ðBÞB À E ðAÞA À E ðBÞB: flowing an aniline–helium gas mixture through a pulsed stab int AB AB A B valve (500 lm diameter). The stagnation pressure was The first term is the corrected interaction energy and the fol- 300 KPa. Aniline was placed in a temperature controlled lowing four terms represent the relaxation contribution, compartment and its partial pressure regulated controlling which compensates for the geometry distortion of the sub- A B its temperature (in a range between À10 °C and 30 °C). In system in the supermolecule, EAB(A) and EAB(B) , with re- A B order to produce aniline–water complexes, the helium was gard to the isolated optimum geometry, EA(A) and EB(B) flowing also through another external compartment con- [24]. Therefore, seven energy calculation had to be carried taining liquid water held at a temperature ranging in an out, instead of the three needed for the determination of 140 G.