ARTICLES Hydrogen-Bonding Interactions of (CF 3 ) 3 CH and (CF 3 ) 3 C - in the Gas Phase. An Experimental (FT-ICR) and Computational Study Andre ´s Guerrero, † Rebeca Herrero, † Juan Z. Da ´valos, † Ivar Koppel, ‡ Jose ´-Luis M. Abboud,* ,† Antonio Chana,* ,† and Ilmar A. Koppel* ,‡ Instituto de Quı ´mica Fı ´sica Rocasolano, CSIC, C/Serrano 119, E-28006 Madrid, Spain, and Institute of Chemistry, Tartu UniVersity, Jakobi Street 2, 51014 Tartu, Estonia ReceiVed: December 15, 2008; ReVised Manuscript ReceiVed: April 15, 2009 Hydrogen-bonding interactions involving 2-(trifluoromethyl)-1,1,1,3,3,3-hexafluoropropane (1H) and 1 - have been quantitatively studied by means of Fourier transform ion cyclotron resonance spectrometry. The existence of the species (1HCl) - and (1H1) - was demonstrated, and their thermodynamic stabilities were determined experimentally and computationally. In addition, some of their structural features were analyzed. I. Introduction We have been involved for some time in the study of the thermodynamic stability of 1:1 hydrogen-bonded complexes of neutral species, both in the gas phase 1 and in solution. 2 The stability of these complexes, formed by interaction of a neutral hydrogen-bond donor (AsH) and a hydrogen-bond acceptor (B), can be measured by the equilibrium constant K p and/or the standard Gibbs energy change for reaction 1 AH(g) + B(g) f AH ··· B(g) K p(1) Δ r G m 0 (1) (1) Numerous 1:1 complexes formed by a neutral molecule (AsH) and anions (B - ) in the gas phase AH(g) + B - (g) f (AH ··· B)(g) K p(2) Δ r G m 0 (2) (2) have been studied by means of (i) mass-spectrometric techniques [notably Fourier transform ion cyclotron resonance spectrometry (FT-ICR) 3 and high-pressure mass spectrometry 4,5 ], (ii) photo- dissociation spectroscopy, 6 (iii) electron photodetachment spec- troscopy, 7 and (iv) multiphoton infrared dissociation spectra. 8 These anionic species include, among others, delocalized carbanions, 9 substituted acetylide ions, 10 and cyanide anion. 11 Intramolecular hydrogen bonds have been reported that also involve carbanionic centers. 12 Reaction 2 has been observed in systems involving CH hydrogen-bond donors and several anions. 13 It has been known for several decades 14 that, in solution, deprotonation of monohydrofluorocarbons (MHFs) of the gen- eral form C n F m H can lead to carbanions of the form C n F m - . The possibility of determining their kinetic acidities using hydrogen/deuterium exchange was also established. This process was later used to compare the kinetic acidities of 2-(trifluoro- methyl)-1,1,1,3,3,3-hexafluoropropane [or tris(trifluoromethyl)- methane C 4 F 9 H(1H)], 1H-perfluorobicyclo[2.2.2]octane (2H), and 1H-perfluorobicyclo[2.2.1] heptane (3H). 15 The fact that the ranking of the rates of hydrogen/deuterium exchange is 1H > 2H . 3H was taken as an indication of the relevance of fluorine (negative) hyperconjugation. The hyper- conjugative effect in 1 - can be visualized as the contribution of the nine possible mesomeric structures shown in Figure 1. They ensure the dispersal of the negative charge over the fluorine atoms C 4 F 9 - T C 4 F 8 F - Internal strain in 2 - and particularly in 3 - reduces this effect and, hence, explains the ranking of kinetic acidities. The gas-phase acidity of an acid AH, GA, is measured by the standard Gibbs energy change for the reaction HA(g) f H + (g) + A - (g) GA ) Δ r G m 0 (3) (3) Using FT-ICR spectrometry, we were able to show 16 that the ranking of thermodynamic acidities of 1H, 2H, and 3H in the gas phase (process 3) is the same as found for their kinetic acidities in solution. This result, being “intrinsic” (solvent- independent), was taken as a strong argument in favor of the negative hyperconjugation of fluorine. This work focuses on 1H. This compound could act as a CH hydrogen-bond donor (HBD). Indeed, the complex between 1H * To whom correspondence should be addressed. E-mail: Jose Luis Abboud: jlabboud@iqfr.csic.es; Antonio Chana: achana@iqfr.csic.es; Ilmar Koppel: ilmar.koppel@ut.ee. † Instituto de Quı ´mica Fı ´sica Rocasolano. ‡ Tartu University. Figure 1. Chemical structures of C 4 F 9 H and C 4 F 9 - . The nomenclature of the different nonequivalent atoms is also depicted. J. Phys. Chem. A 2009, 113, 6422–6429 6422 10.1021/jp811057m CCC: $40.75  2009 American Chemical Society Published on Web 05/19/2009