Permanent Electric Dipole of Gas-Phase p-Amino Benzoic Acid Isabelle Compagnon, Rodolphe Antoine, Driss Rayane, Michel Broyer, and Philippe Dugourd* Laboratoire de Spectrome ´ trie Ionique et Mole ´ culaire, UMR n°5579, CNRS et UniVersite ´ Lyon 1, ba ˆ t Alfred Kastler, Campus de la Doua, 69622 Villeurbanne Cedex, France ReceiVed: October 17, 2002; In Final Form: February 13, 2003 By coupling a matrix-assisted laser desorption source to an electric beam deflection setup, we have measured the permanent electric dipole moment of the isolated p-amino benzoic acid (PABA) molecule in the ground state along its principal axis. This is the first measurement of the electric dipole of an isolated push-pull molecule. The experimental value is compared to different calculations, and the accuracy of Hartree-Fock and density functional approaches to predict the electronic properties of π-conjugated systems is discussed. Introduction Experimental and theoretical interests in π-conjugated mol- ecules have exploded with the possibility of designing and synthesizing molecules with specific electronic and optical properties and their potential applications as constituents of new electric or optical devices. 1-4 For years, doubly substituted benzene molecules have served as an important model for conjugated systems, in particular for dipolar push-pull mol- ecules. 3,5 In a push-pull molecule, a donor (D) and an acceptor (A) groups are connected together via a π-conjugated system, here the phenyl ring. The linear and non linear optical properties of these molecules are strongly related to the internal charge transfer between the two groups and to the modification of this transfer between the ground and excited states. 3,4,6,7 Detailed experimental results are available on the electronic and optical properties of these molecules in solution, but very few results are available on gas-phase molecules. This lack of experimental results for isolated molecules makes difficult the comparison with theoretical predictions. Indeed, the solvent induces modi- fications in the electronic and optical properties of the molecule, in particular a shift in the optical absorption spectrum. Recently, different approaches have been developed to incorporate solvent effects into the simulation (see, for example, references 8-13). However, the calculation for molecules in a solvent is still less advanced with respect to accuracy and reliability than that in the gas phase. In parallel to the improvement of calculations in the condensed phase, experimental results in the gas phase are needed for a better understanding of the fundamental properties of these molecules. In this article, we present the first measure- ment of the permanent electric dipole of a gas-phase doubly substituted benzene molecule: the p-amino benzoic acid mol- ecule (PABA). The permanent dipole is a direct probe of the internal charge transfer in the ground state and its determination is important to understand the electronic and optical properties of the molecule. Moreover, comparison between gas-phase and condensed-phase measurements may allow a better description of the effects of a solvent on a dipolar push-pull molecule. More generally, electric dipole measurements may provide experimental data to assess the validity of theoretical methods to compute the electronic properties of a π-conjugated system. Different calculations are compared to experimental results in the last part of this article. Experimental Section The apparatus consists of a matrix-assisted laser desorption source (MALD) coupled to an electric beam deflection setup and a position-sensitive time-of-flight mass spectrometer. 14 PABA is purchased from SIGMA (purity > 99%) and mixed with cellulose in a 1:10 mass ratio. The sample is pressed under 10 4 bars in a cylindrical mold to form a solid rod. The rod is placed into the source where the molecules are desorbed from the cellulose matrix with the third harmonic of a Nd 3+ :YAG laser (355 nm). The molecules are entrained out by a helium flush supplied by a pulsed valve that is synchronized with the desorption laser shot. The molecules leave the source through a 5 cm long nozzle in which the temperature can be adjusted. Otherwise stated, all the experiments described in this article were performed at room temperature. The beam is collimated by two skimmers and two slits. Its velocity is selected and measured with a mechanical chopper located in front of the first slit. Then, it travels through a 15 cm long electric deflector. The deflecting field is produced using a “two-wire” field. The electric field F and the field gradient ∂F/∂z are perpendicular to the axis of the beam and are nearly constant over its width. 15 One meter after the deflector, the molecules are two-photon ionized with the fourth harmonic of a Nd 3+ :YAG (266 nm) and mass-selected with a time-of-flight mass spectrometer. We use a position-sensitive time-of-flight. 16 The profile of the beam in the ionization region is determined from the arrival time distribution at the detector. The beam profile is measured as a function of the electric field in the deflector. Results Figure 1 shows a mass spectrum of PABA recorded at room temperature. The dominant peak is observed at the parent mass 137 and is attributed to the PABA molecule. The small peak observed at mass 138 is due to the isotopic distribution. The source conditions were adjusted in order that no dimer was produced. Small fragmentation peaks are also observed at mass 120 and mass 93. They correspond to OH and COO losses occurring during the two-photon ionization process. The gas- phase PABA molecule was first studied by G. Meijer and * Author to whom correspondence should be addressed. E-mail: dugourd@lasim.univ-lyon1.fr. 3036 J. Phys. Chem. A 2003, 107, 3036-3039 10.1021/jp022247j CCC: $25.00 © 2003 American Chemical Society Published on Web 04/08/2003