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 MalononitrileDaniel J. Goebbert, Luis Velarde,† Dmitry Khuseynov, and Andrei Sanov*

Department of Chemistry and Biochemistry, University of 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 affinityof the dicyanomethyl radical, •CH(CN)2, EA[

•CH(CN)2]=2.88 ( 0.01 eV, measuredby photoelectron imaging of the CH(CN)2

- anion, and the acidity/electron affinitythermodynamic cycle, we obtained the C-H bond dissociation enthalpy of malo-nonitrile, CH2(CN)2, DH298[H-CH(CN)2]=87( 2 kcal/mol. This result is comparedto the corresponding value for acetonitrile, DH298(H-CH2CN)=93 ( 2 kcal/mol,determined from a similar measurement of EA(•CH2CN) = 1.53 ( 0.01 eV. Therelativeweakness of the C-Hbonds inmalononitrile and acetonitrile, compared tomost closed-shell neutral organic molecules, is attributed to π-resonance stabiliza-tion of the unpaired electrons in •CH(CN)2 and

•CH2CN.

SECTION Kinetics, Spectroscopy

D ue to its large electronegativity, the CN group is oftenregarded as a pseudohalogen. However, halogens areπ-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 ofmalononitrile, CH2(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 (CH4), acetonitrile (CH3CN), andcyanoform, CH(CN)3. This CN-substituted series is in turn com-pared to halogenated methanes, such as CHnFm (nþm=4).We find a clear contrast in how the C-H bond energies areaffected by the π-donating halogens compared to the reso-nance stabilization offered by the CN group.

The 355 nm photoelectron image and the correspondingspectrum for CH(CN)2

- are shown in Figure 1a. For compa-rison, the photoelectron image and the corresponding spec-trum of CH2CN

-, measured at 532 nm, are displayed inFigure 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 typeorbitals.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.Calculations4 at theB3LYP/aug-cc-pVDZ level of theory predictan EA of 2.92 eV, in good agreement with the experimentalvalue.

The CH2CN- spectrum shows a short vibrational progres-

sion with a sharp origin, corresponding to an electron affinityof 1.53( 0.01 eV. This spectrum is in good agreement with aprevious study of CH2CN

-, which yielded an EA of 1.543 (0.014.5 The most intense spectral peak in Figure 1b is thetransition origin, while all other bands (spaced by∼700 cm-1)

correspond to the excitation of the umbrellamode of •CH2CN.5

Our B3LYP/6-311þþG(3df,3pd) calculations4 on •CH2CN pre-dict an umbrella mode frequency of 684 cm-1 and electronaffinity of 1.57 eV, in good agreement with the experimentaldata.

The comparison of the CH(CN)2- and CH2CN

- photoelec-tron spectra in Figure 1 highlights the absence of a discerniblevibrational progression in the CH(CN)2

- case. This observa-tion suggests that the CH(CN)2

- anion and the •CH(CN)2neutral must have very similar geometries. To support thisconclusion, we optimized the geometries of CH(CN)2

- and•CH(CN)2 at the B3LYP/aug-cc-pVDZ level of theory.4 Theresulting structures are shown in Figure 2. Both CH(CN)2

-

and •CH(CN)2 are predicted to have planar structures of C2vsymmetry, with a 1A1 electronic state for the anion and a 2B1ground state for the neutral radical. The anion and neutralstructures are indeed essentially identical at this level oftheory, with the most noticeable difference being in the CCCbond angle, which is predicted to decrease by 1.2� uponelectron detachment.

In contrast, CH2CN- is known to be nonplanar (Cs sym-

metry), while the corresponding neutral radical has a planarstructure.5 This geometry difference is responsible for thevibrational progression in the photoelectron spectrum ofCH2CN

- seen in Figure 1b. The striking difference betweentheplanargeometryofCH(CN)2

- and thenonplanar structureof CH2CN

- is attributed to decreased electron density onthe central carbonatom in thepresenceof twoCNgroups. TheMulliken analysis of CH(CN)2

- in Figure 2 indicates a largepositive charge on the central carbon, which favors a planargeometry. A similar calculation for CH2CN

- shows a negative

Received Date: December 8, 2009Accepted Date: January 28, 2010

rXXXX American Chemical Society 793 DOI: 10.1021/jz900379t |J. Phys. Chem. Lett. 2010, 1, 792–795

pubs.acs.org/JPCL

Mulliken charge (-0.536) on the central carbon. To put therelationship between geometry and charges in perspective,the carbon in CH3

þ is positively charged and the ion is planar,while in CH3

-, the carbon is negatively charged and the anionis nonplanar. While these arguments are not new,6 this subtleresult highlights inductive effects typically associated with thepseudohalogen (electronegative) nature of the CN group,which is electron-withdrawing in the σ system.

The experimental electron affinity of •CH(CN)2 allows us tocalculate the C-H bond enthalpy (DH298) of malononitrileusing the general acidity/electron affinity cycle1

DH298ðR-HÞ ¼ ΔacidH298ðRHÞþ EAðR•Þ- IEðHÞþ ½thermal correction� ð1Þ

In this equation, ΔacidH298(RH) is the gas-phase acidity of aclosed-shell molecule, EA(R•) is the electron affinity of thecorresponding radical, and IE(H) = 315.1 kcal/mol is theionization energy of atomic hydrogen.7 The small thermalcorrection is a set of heat capacity integrals from 0 to 298 K,corresponding to RH, R-, H, andHþ. This correction is usuallysmaller than 0.3 kcal/mol and is hence absorbed by largerexperimental uncertainties.8 Neglecting the [thermal correc-tion] term in eq 1 and using the known acidity of malononi-trile, ΔacidH298[CH2(CN)2]=335.8 ( 2.1 kcal/mol,9 and theexperimental EA of dicyanomethyl radical, EA[•CH(CN)2] =2.88 ( 0.01 eV, we find the C-H bond dissociation energyofmalononitrile to be DH298[H-CH(CN)2]=87( 2 kcal/mol.A similar calculation for acetonitrile, withΔacidH298(CH3CN)=372.9( 2.1 kcal/mol (from ref 10) and EA(•CH2CN)=1.53(0.01 eV (asdeterminedhere), givesDH298(H-CH2CN)=93(2 kcal/mol. This result is within the uncertainty of the pre-viously reported value, 94.2 ( 2.0 kcal/mol.5

The so determined bond dissociation energy of malo-nonitrile, 87 ( 2 kcal/mol, is small compared to typicalC-H bond energies of neutral closed-shell organicmolecules.In general, bond energies reflect the stability of the corre-sponding radicals; smaller DH298 values translate into greaterradical stability. As a reference, the bond dissociation energyof methane, DH298(H-CH3)=104.9 ( 0.4 kcal/mol,1 is sig-nificantly larger than the corresponding values for acetonitrileand malononitrile. In the methyl radical, the unpaired elec-tron is localized on the carbon atom and receives no addi-tional stabilization. For comparison, the bond energiesof molecules producing “stable” radicals, such as toluene,DH298(H-CH2C6H5) = 89.8 ( 0.6 kcal/mol, and propene,DH298(H-CH2CHCH2) = 88.8 ( 0.4 kcal/mol, are similarto those of acetonitrile and malononitrile.1 Compared to thatof methane, the lower DH298(H-CH2CN) of acetonitrilereflects stabilization of the unpaired electron in the cyano-methyl radical through π resonance with the CN group. Inthe dicyanomethyl radical, the additional CN group furtherstabilizes the unpaired electron via increased resonanceinteraction, resulting in the even lowerC-Hbonddissociationenergy of the parent molecule, malononitrile.

Although the CN group is often regarded as a pseudohalo-gen, radical stabilization in •CH2CNand •CH(CN)2 is not due toelectron-withdrawing properties of CN, which it shares withatomic halogens, but is manifested in the π system. Similarhalogenated methanes are not resonance-stabilized as halo-gens act as π-donors. For the case of H-CHnFm (n þ m=3),we find DH298(H-CH2F) = 100 ( 6 kcal/mol, DH298(H-CHF2) = 102 ( 5 kcal/mol, and DH298(H-CF3) = 104 (2 kcal/mol;11 thus, the bond energies increasewith sequentialhalogen substitution.

While the DH298 difference between CH4 and CH3CN isabout14kcal/mol, thedifferencebetweenCH3CNandCH2(CN)2

Figure 1. Photoelectron images and corresponding spectra of(a) CH(CN)2

-, measured at 355 nm, and (b) CH2CN-, measured

at 532 nm. The composite images show the raw (left side) andreconstructed data (right). The double arrows indicate the laserpolarization direction.

Figure 2. Geometries of CH(CN)2- and CH(CN)2 optimized at the

B3LYP/aug-cc-pVDZ level of theory. Both structures are planar andbelong to the C2v symmetry point group. Bond lengths are inAngstroms. Values in parentheses are Mulliken charges.

rXXXX American Chemical Society 794 DOI: 10.1021/jz900379t |J. Phys. Chem. Lett. 2010, 1, 792–795

pubs.acs.org/JPCL

is about 6 kcal/mol, indicating a moderate increase in reso-nance stabilization with the addition of the second CN group.On the basis of this trend, we hypothesize that cyanoform,CH(CN)3, should have an even smaller C-H bond dissocia-tion energy because of additional resonance stabilization inthe resulting radical, •C(CN)3. While difficult to isolate,12,13

CH(CN)3 may have one of the weakest C-H bonds for aclosed-shell neutral organic molecule.

Finally, we calculate the heats of formation of the •CH2CNand •CH(CN)2 radicals

1

ΔfH298ðR•Þ ¼ DH298ðRHÞ þ ΔfH298ðRHÞ-ΔfH298ðHÞ ð2ÞUsingΔfH298(H)=52.103(0.003kcal/mol,14ΔfH298[CH2(CN)2]=63.64 ( 0.24 kcal/mol,15 and the C-H bond dissocia-tion energy of malononitrile determined here, we findΔfH298[

•CH(CN)2]=99 ( 2 kcal/mol. For comparison, usingΔfH298(CH3CN) = 17.70 ( 0.09 kcal/mol16 and our valueof DH298(H-CH2CN), we find ΔfH298(

•CH2CN) = 59 (2 kcal/mol, in good agreement with the previous studies,which yielded 59.7 ( 2.05 and 58.5 ( 2.2 kcal/mol.17

In summary, the electron affinity of the dicyanomethylradical, EA=2.88( 0.01 eV, wasmeasured by photoelectronimaging of the CH(CN)2

- anion. This result was used toobtain the C-H bond dissociation energy of malononitrile,DH298[H-CH(CN)2] = 87 ( 2 kcal/mol, and the heat offormation of the resulting radical, ΔfH298[

•CH(CN)2]=99 (2 kcal/mol. The C-H bond dissociation energy of malononi-trile is smaller than that of most closed-shell neutral organicmolecules, including methane and acetonitrile. The bondweakness is attributed to resonance stabilization of theunpaired electron in •CH(CN)2.

EXPERIMENTAL METHODS

The electron affinities of •CH(CN)2 and •CH2CN weremeasuredusing a previously describednegative ion18 velocitymap19 photoelectron imaging20 spectrometer.21 The CH-(CN)2

- and CH2CN- ions were generated by the proton-

abstraction reaction of O- with malononitrile or acetonitrile,respectively. Mass-selected anions were photodetached usingthe third (355 nm) or second (532 nm) harmonic of a Nd:YAG(yittrium aluminum garnet) laser with pulse energies of10-20 mJ and an 8 ns pulse width. Photoelectron imageswere analyzed following published procedures.22 The photo-electron spectra were calibrated using the known electronaffinity of O-.23

AUTHOR INFORMATION

Corresponding Author:*To whom correspondence should be addressed. E-mail: sanov@u.arizona.edu.

Present Addresses:†Department of Chemistry, University of California, Santa Barbara,CA 93106.

ACKNOWLEDGMENT This work was funded by the U.S. NationalScience Foundation through Grant CHE-0713880.

REFERENCES

(1) Blanksby, S. J.; Ellison, G. B. Bond Dissociation Energies ofOrganic Molecules. Acc. Chem. Res. 2003, 36, 255–263.

(2) Mabbs, R.; Grumbling, E. R.; Pichugin, K.; Sanov, A. Photo-electron Imaging: An Experimental Window into ElectronicStructure. Chem. Soc. Rev. 2009, 38, 2169–2177.

(3) Sanov, A.; Mabbs, R. Photoelectron Imaging of Negative Ions.Int. Rev. Phys. Chem. 2008, 27, 53–85.

(4) Frisch, M. J.; et al. Gaussian 03, revision B.01; Gaussian, Inc.:Wallingford, CT, 2004.

(5) Moran, S.; Ellis, H. E., Jr.; DeFrees, D. J.;McLean, A.D.; Ellison,G. B. Carbanion Spectroscopy: CH2CN

-. J. Am. Chem. Soc.1987, 109, 5996–6003.

(6) Bickelhaupt, F. M.; Ziegler, T.; Schleyer, P. v. R. CH3• is Planar

Due to H-H Steric Repulsion. Theoretical Study of MH3• and

MH3Cl (M=C, Si, Ge, Sn). Organometallics 1996, 15, 1477–1487.

(7) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.;Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. The NBSTables of Chemical Thermodynamic Properties (NBS TechNote 270). J. Phys. Chem. Ref. Data 1982, 11, 1.

(8) Ervin, K. M. Experimental Techniques in Gas-Phase IonThermochemistry. Chem. Rev. 2001, 101, 391–444.

(9) Fujio, M.; McIver, R. T.; Taft, R. W. Effects on the Acidities ofPhenols from Specific Substituent-Solvent Interaction: In-herent Substituent Parameter fromGas PhaseAcidities. J. Am.Chem. Soc. 1981, 103, 4017–4029.

(10) Bartmess, J. E.; Scott, J. A.; McIver, R. T. Scale of Acidities inthe Gas-Phase from Methanol to Phenol. J. Am. Chem. Soc.1979, 101, 6046–6056.

(11) Bartmess, J. E. In NIST Chemistry WebBook, NIST Stan-dard Reference Database Number 69; Linstrom, P. J., Mallard,W. G., Eds.; National Institute of Standards and Technology:Gaithersburg, MD, 2009; http://webbook.nist.gov (retrievedJanuary 12, 2010).

(12) Bak, B.; Svanholt, H. Existence of Gaseous Cyanoform asObserved by Microwave Spectra. J. Mol. Struct. 1977, 37,153–156.

(13) Bak, B.; Bjorkman, C. Ab Initio Approach to the CyanoformProblem. J. Mol. Struct. 1975, 25, 131–140.

(14) Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA KeyValues for Thermodynamics; Hemisphere Publishing Corp.:New York, 1984.

(15) Beckhaus, H. D.; Dogan, B.; Pakusch, J.; Verekin, S.; Ruechardt,C. Geminal Substituent Effects. 4. The Dependence of theInverse Anomeric Effect of Geminal Cyano Groups on theStructure. Chem. Ber. 1990, 123, 2153–2159.

(16) An, X.; Mansson, M. Enthalpies of Combustion and For-mation of Acetonitrile. J. Chem. Thermodyn. 1983, 15, 287–293.

(17) King, K. D.; Goddard, R. D. Very Low-Pressure Pyrolysis ofAlkyl Cyanides. 2. N-Propyl Cyanide and Isobutyl Cyanide:Heat of Formation and Stabilization Energy of CyanomethylRadical. Int. J. Chem. Kinet. 1975, 7, 837–855.

(18) Johnson,M. A.; Lineberger,W. C. In Techniques for the Study ofIon Molecule Reactions; Farrar, J. M., Saunders, W. H., Eds.;Wiley: New York, 1988; pp 591-635.

(19) Eppink, A.; Parker, D. H. Velocity Map Imaging of Ions andElectrons Using Electrostatic Lenses: Application in Photo-electron and Photofragment Ion Imaging of Molecular Oxy-gen. Rev. Sci. Instrum. 1997, 68, 3477–3484.

(20) Chandler, D. W.; Houston, P. L. Two-Dimensional Imaging ofState-Selected Photodissociation Products Detected byMulti-photon Ionization. J. Chem. Phys. 1987, 87, 1445–1447.

rXXXX American Chemical Society 795 DOI: 10.1021/jz900379t |J. Phys. Chem. Lett. 2010, 1, 792–795

pubs.acs.org/JPCL

(21) Velarde, L.; Habteyes, T.; Sanov, A. Photodetachment andPhotofragmentation Pathways in the [(CO2)2(H2O)m]

- Clus-ter Anions. J. Chem. Phys. 2006, 125, 114303.

(22) Dribinski, V.; Ossadtchi, A.; Mandelshtam, V. A.; Reisler, H.Reconstruction of Abel-Transformable Images: The GaussianBasis-Set Expansion Abel Transform Method. Rev. Sci. In-strum. 2002, 73, 2634–2642.

(23) Neumark, D. M.; Lykke, K. R.; Andersen, T.; Lineberger, W. C.Laser Photodetachment Measurement of the Electron Affi-nity of Atomic Oxygen. Phys. Rev. A 1985, 32, 1890–1892.