rXXXX American Chemical Society 792 DOI: 10.1021/jz900379t | J. Phys. Chem. Lett. 2010, 1, 792–795 pubs.acs.org/JPCL C-H Bond Dissociation Energy of Malononitrile Daniel J. Goebbert, Luis Velarde, † Dmitry Khuseynov, and Andrei Sanov* Department of Chemistry and Biochemistry, Universityof Arizona, Tucson, Arizona 85721-0041 ABSTRACT The C-H bond dissociation energies of closed-shell molecules de- crease with increasing stability of the resulting radicals. From the electron affinity of the dicyanomethyl radical, • CH(CN) 2 , EA[ • CH(CN) 2 ] = 2.88 ( 0.01 eV, measured by photoelectron imaging of the CH(CN) 2 - anion, and the acidity/electron affinity thermodynamic cycle, we obtained the C-H bond dissociation enthalpy of malo- nonitrile, CH 2 (CN) 2 , DH 298 [H-CH(CN) 2 ] =87 ( 2 kcal/mol. This result is compared to the corresponding value for acetonitrile, DH 298 (H-CH 2 CN) = 93 ( 2 kcal/mol, determined from a similar measurement of EA( • CH 2 CN) = 1.53 ( 0.01 eV. The relative weakness of the C-H bonds in malononitrile and acetonitrile, compared to most closed-shell neutral organic molecules, is attributed to π-resonance stabiliza- tion of the unpaired electrons in • CH(CN) 2 and • CH 2 CN. SECTION Kinetics, Spectroscopy D ue to its large electronegativity, the CN group is often regarded as a pseudohalogen. However, halogens are π-donors, while the CN group is not and acts more like an aryl group in an extended π system. This critical dis- tinction is revealed in the C-H bond energies of the corres- ponding substituted methanes. 1 We report the gas-phase C-H bond dissociation energy of malononitrile, CH 2 (CN) 2 , determined from the electron affi- nity of the corresponding dicyanomethyl radical, • CH(CN) 2 . The results are discussed in comparison with the correspond- ing properties of methane (CH 4 ), acetonitrile (CH 3 CN), and cyanoform, CH(CN) 3 . This CN-substituted series is in turn com- pared to halogenated methanes, such as CH n F m (n þ m =4). We find a clear contrast in how the C-H bond energies are affected by the π-donating halogens compared to the reso- nance stabilization offered by the CN group. The 355 nm photoelectron image and the corresponding spectrum for CH(CN) 2 - are shown in Figure 1a. For compa- rison, the photoelectron image and the corresponding spec- trum of CH 2 CN - , measured at 532 nm, are displayed in Figure 1b. In both cases, the photoelectron angular distribu- tions peak in the direction perpendicular to the laser polari- zation axis, characteristic of detachment from carbon 2p type orbitals. 2,3 For CH(CN) 2 - , we observe a single, remarkably narrow band at an electron binding energy of eBE = 2.88 ( 0.01 eV, which corresponds to the electron affinity (EA) of • CH(CN) 2 . Calculations 4 at the B3LYP/aug-cc-pVDZ level of theory predict an EA of 2.92 eV, in good agreement with the experimental value. The CH 2 CN - spectrum shows a short vibrational progres- sion with a sharp origin, corresponding to an electron affinity of 1.53 ( 0.01 eV. This spectrum is in good agreement with a previous study of CH 2 CN - , which yielded an EA of 1.543 ( 0.014. 5 The most intense spectral peak in Figure 1b is the transition origin, while all other bands (spaced by ∼700 cm -1 ) correspond to the excitation of the umbrella mode of • CH 2 CN. 5 Our B3LYP/6-311þþG(3df,3pd) calculations 4 on • CH 2 CN pre- dict an umbrella mode frequency of 684 cm -1 and electron affinity of 1.57 eV, in good agreement with the experimental data. The comparison of the CH(CN) 2 - and CH 2 CN - photoelec- tron spectra in Figure 1 highlights the absence of a discernible vibrational progression in the CH(CN) 2 - case. This observa- tion suggests that the CH(CN) 2 - anion and the • CH(CN) 2 neutral must have very similar geometries. To support this conclusion, we optimized the geometries of CH(CN) 2 - and • CH(CN) 2 at the B3LYP/aug-cc-pVDZ level of theory. 4 The resulting structures are shown in Figure 2. Both CH(CN) 2 - and • CH(CN) 2 are predicted to have planar structures of C 2v symmetry, with a 1 A 1 electronic state for the anion and a 2 B 1 ground state for the neutral radical. The anion and neutral structures are indeed essentially identical at this level of theory, with the most noticeable difference being in the CCC bond angle, which is predicted to decrease by 1.2° upon electron detachment. In contrast, CH 2 CN - is known to be nonplanar (C s sym- metry) , while the corresponding neutral radical has a planar structure. 5 This geometry difference is responsible for the vibrational progression in the photoelectron spectrum of CH 2 CN - seen in Figure 1b. The striking difference between the planar geometry of CH(CN) 2 - and the nonplanar structure of CH 2 CN - is attributed to decreased electron density on the central carbon atom in the presence of two CN groups. The Mulliken analysis of CH(CN) 2 - in Figure 2 indicates a large positive charge on the central carbon, which favors a planar geometry. A similar calculation for CH 2 CN - shows a negative Received Date: December 8, 2009 Accepted Date: January 28, 2010