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J . Phys. Chem. 1992, 96, 1662-1667
valence charge density to its bonded N and 0 atoms by exhibiting, in addition to the three bonded charge concentrations in the plane of the amide nuclei, regions of charge depletion on each side of this plane. The positions of these regions of charge depletion or quite literally “holes” in the VSCCs of the amide carbon atoms are located at the (3,+1) critical points in -V2p and they serve as the sites of nucleophilic attack in the hydrolysis of the amide bond. The position of this critical point in GIG’ defines an angle of approach relative to the C - 0 axis of a nucleophile to the amide carbon equal to 113O, in agreement with the structure correlation result of Burgi and dun it^^^ obtained from observed crystal structures. The methylenic carbon possesses four tetrahedrally oriented bonded charge concentrations, two of which are in the plane of the amide nuclei. Chemically equivalent atoms exhibit similar values whether in the glycine or alanine groups. The basicity of a given atom parallels the magnitude of the local maximum in -V2p determined at a (3,-3) critical point. Thus the keto oxygens are protonated in preference to the nitrogens of the amide bonds, as is true in the protonation of f ~ r m a m i d e . ~ ~The , * ~nonbonded charge concentrations on the hydroxyl oxygens of the carboxylic acid groups in G’and A’ are also of lesser magnitude than those on the keto oxygens. There are significant differences in the magnitudes of the two nonbonded charge concentrations on the keto oxygen atoms, indicating a preferred site of protonation syn or anti to a neighboring atom. The magnitude of the single nonbonded charge concentration on the nitrogen of the terminal amino group in the G and A groups is larger than that for the two equivalent such charge concentrations on the amide nitrogens in G’,A’, and G”. The positions as well as the magnitudes of V 2 pat the critical points in the VSCCs of the peptide atoms differ by only small amounts throughout the series of molecules. The magnitudes of V 2 pat the nonbonded in-plane maxima in the VSCCs of N 3 in GI and AI and of 0 4 in all groups vary by less than 1% while the out-of-plane maxima on N 3 in fragments IG”l, IG’, and IA’ vary by less than 2%. The magnitudes of V 2 p at the out-of-plane
minima in the VSCC of an amide carbon C2 in GI, G”(,and A( which determine the pasitions of nucleophilic attack in the cleavage of the C-N bond, vary by less than 0.009 au. The susceptibility to attack increases with the size of the hole, Le., with the magnitude of V 2 p ,and the differences in these values, while small, are s i g nificant. Thus an amide bond to a terminal -NH2 group is predicted to be more reactive than an interior peptide bond for all three fragments. The relative magnitudes of the two nonbonded charge concentrations on a keto oxygen are preserved in the synthesized molecules. The Laplacian distribution for synthesized GIG’ correctly predicts the nonbonded charge concentration on the keto oxygen of G’I anti to the amide hydrogen and a like concentration on the keto oxygen of GI anti to NH2 to be the favored sites of protonation on these atoms. Corresponding results are obtained for the tripeptide. This type of information that is so readily read from a Laplacian map is of direct use in predicting relative reactivities of active sites.
Discussion The secondary structure resulting from the linking of four or more peptide groups, as described here, causing changes in the dihedral angles $ and 4 must initially be dealt with in an empirical manner, using experimental geometries. There is no need to match any interatomic surfaces to use the knowledge of the atomic and group properties for the peptide fragments, as defined here, together with the experimental geometry to predict the van der Waals shapes using actual atomic density envelopes, to determine electrostatic fields using the transferable group moments, and to complement these fields using the more detailed information contained in the properties of the Laplacian of the charge distribution. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We also acknowledge access to the CRAY computer at OCLSC, Toronto. Registry No. H-Gly-NH2, 598-41-4; OHC-Gly-NH2, 4238-57-7; OHC-Gly-OH, 2491-15-8; OHC-Ala-OH, 10512-86-4; H-Ala-NH,, 7324-05-2; H-Gly-Gly-OH, 556-50-3; H-(GIy),-OH, 556-33-2; H-GlyAla-OH, 3695-73-6; H-Ala-Gly-OH, 687-69-4.
(24) Burgi, H. B.; Dunitz, J. D. Acc. Chem. Res. 1983, 16, 153. (25) Birchall, T.; Gillespie, R. J. Can. J . Chem. 1963, 41, 2642. (26) Gandour, R. Bioorg. Chem. 1981, IO, 169.
Threshold Collisional Activation of FeC,H,+:
Fe+*Ethane vs Fe+*Dimethyl Structures
Richard H. Schultzt and P. B. Armentrout*,l Department of Chemistry, University of Utah, Salt Lake City, Utah 841 12 (Received: August 26, 1991; In Final Form: October 24, 1991) Threshold collisional activation (TCA) of FeC2H6+is studied in a guided-ion beam mass spectrometer. Parent ions are formed by reaction of Fe+ with either ethane or acetone in a flow tube ion source which ensures their thermalization. We present
evidence that ions formed in the two ways have different structures correspondingto Fe+-ethane and Fe+-dimethyl,respectively. We determine Doo(Fe+-C2H6)= 15.3 1.4 kcal/mol and Doo(CH,Fe+-CH3) = 43.1 f 2.6 kcal/mol and that Fe(C&)+ is more stable than Fe(CH3)2+by 3.5 f 1.7 kcal/mol. These experimental values are compared with those from a recent theoretical calculation. The utility of TCA as a probe of thermochemistry and the structure of transition metal-alkane and -dialkyl species is assessed.
*
Introduction One topic of much current study is the interactions of transition metals with alkanes, in particular, how C-C and C-H bond activation occurs. These systems have been investigated by a wide variety of techniques.I The simplest metal/alkane system in which both C-C and C-H bond activation can be observed is the reaction of a bare atomic metal (neutral or ionic) with ethane. Ionic species ‘Present address: Department of Chemistry, University of California, Berkeley, CA 94720. *Camille and Henry Dreyfus Teacher-Scholar, 1987-1992.
0022-365419212096-1662$03.00/0
have the advantages that they can be produced relatively easily, their energy controlled precisely, and their identification by mass ( I ) Shilov, A. E. Actiuation of Saturated Hydrocarbons by Transition Metal Complexes; Reidel: Dordrecht, The Netherlands, 1984. Crabtree, R. H. Chem. Rev. 1985, 85, 245. Actiuation and Functionalization of Alkanes; Hill, C . L., Ed.; Wiley-Interscience: New York, 1989. Gas Phase Inorganic Chemistry; Russell, D. H.. Ed.; Plenum: New York, 1989. Bonding Energetics in Organometallic Compounds; Marks, T. J., Ed.; American Chemical Society: Washington, DC. 1990. Selectiue Hydrocarbon Actiuation: Principles and Progress: Davies, J. A., Watson, P. L., Liebman, J. F., Greenberg, A,, Ed.; VCH: New York, 1990.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96,No. 4, 1992 1663
Threshold Collisional Activation of FeC2H6+ TABLE I: Literature Thermochemistry Used in This Study" species CH3 C2H6
(CH3)2C0
AiHao9 kcal/mol 35.6 -16.4b
AiH'zm kcal/mol 34.8 -20.1b -51.9b
species
AiHOo, kcal/mol
co Fe Fe+
98.7 281.OC
AiHaz9a, kcal/mol -26.4 99.3 283.W
"Unless otherwise stated, values in this table are taken from: Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J . Phys. Chem. Ref. Data 1985,14, Supp. No. 1 (JANAF Tables). bLias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref. Data 1988, 17, Supp. No. 1. CIE(Fe) taken from Sugar, J.; Corliss, C. J . Phys. Chem. Ref. Data 1985, 14, Supp. No. 2.
spectrometric techniques relatively straightforward. Thus, study of transition metal ion/alkane systems has been actively pursued because of the wealth of detailed thermodynamic and mechanistic information that can often be obtained. In the specific case of transition metal ions interacting with ethane, a question of interest in these systems is whether an M(CH3)2+ion produced by metal insertion into the ethane C-C bond is more or less stable than an M+.ethane adduct ion. Recently, Rosi et a1.2 have reported results of an ab initio study that calculated the energetics of M(CH3)z+ions for the first and second row transition metals. They reported that, for groups 3 and 4, the dimethyl structure is clearly preferred and that, for groups 6 and 11, the adduct ion is the preferred structure. For the remaining transition metal ions, their calculations were insufficiently precise to determine the relative stabilities of the two structures. There is only a limited amount of experimentally derived information currently available on this problem. Prior work from our laboratory has used thermochemical measurements of endothermic bimolecular ion-molecule reactions to determine Do(CH3M+-CH3)for SC+,~ Ti+: and V+.5 While these three ions all form dimethyl ions from endothermic reactions with hydrocarbons, the second methyl bond dissociation energy cannot be measured in this way for any other first row transition metals, as they do not undergo such endothermic reactions. However, Fe+,6 Co+,'and Ni+' are observed to exothermically decarbonylate acetone, establishing that Do(M+-CH3) + Do(CH3M+-CH3)2 95 kcal/mol (Table I). In agreement with this limit, Hanratty et a1.8 obtained 105 f 5 kcal/mol for the sum of the metal-methyl bond energies in C O ( C D ~ )by ~ +investigating the kinetic energy release distributions from the reaction of Co+ with acetone-d6. (Alternatively, this data can be interpreted in terms of a Co+.C2D6 structure and provides a Co+-C2D6bond energy of -23 kcal/moL9 Recent work in our laboratory explicitly demonstrates that the latter proposition is correct and measures D0,)(Co+
