4408
J. Phys. Chem. 1996, 100, 4408-4412
Calculation of the Ionization Energies of the Amidogen and Methyl-Substituted Amidogen Radicals: NH2, CH3NH, and CH3NCH3 Timothy G. Wright† and Terry A. Miller* Laser Spectroscopy Facility, Chemistry Department, The Ohio State UniVersity, 120 West 18th AVenue, Columbus, Ohio 43210 ReceiVed: September 13, 1995; In Final Form: December 18, 1995X
Despite considerable attention over the years, the claimed ionization energies of the methyl- and dimethylsubstituted amidogen radicals are surprisingly diverse. In this paper, the available evidence is reviewed and the controversies are highlighted. New ab initio calculations have been performed, using the ACES2 package and the G1 and G2 methodologies, and evaluated for the amidogen radical, NH2. It is concluded that the inexpensive G1 and G2 calculations, including scaling, allow a confident prediction of the ionization energies of CH3NH (9.8 ( 0.1 eV) and CH3NCH3 (9.1 ( 0.1 eV).
I. Introduction As long ago as the early 1900s, the existence of dialkylamidogen radicals (also known as dialkylnitrene radicals and dialkylaminyl radicals) was known.1 The family of alkyl- and arylamidogen radicals has been noted to be involved in cancer formation.2 Alkyl- and arylamidogen ions are important intermediates in organic chemistry,3 and reviews are available.4 The radicals have also been invoked as ligands in organometallic chemistry.5 Much work has been performed to try to establish the ionization energies of the three simplest members of the alkylamidogen group: NH2, CH3NH, and CH3NCH3sknown as the amidogen radical, monomethylamidogen (MMA), and dimethylamidogen (DMA), respectively. Although the ionization energy of the amidogen radical seems fairly well established as 11.14 ( 0.01 eV,6 the values quoted for MMA and DMA in two important compilations seem to be in sharp disagreement. The 1977 summary7 of ionization energies and heats of formation of ions give a range of appearance potentials for a CH4N+ ion, noted as probably CH2NH2+. These range from 9.5 to 13.1 eV. However, the equivalent 1988 volume8 recommends an ionization energy of 6.7 eV for the CH3NH isomer and 6.1 eV for the CH2NH2 isomer. The former value is calculated from heats of formation of the neutral and the ion, whereas the latter is an appearance potential. In a similar way, for C2H6N+, the 1977 compilation gives appearance potentials ranging from 9.0 to 13.7 eV, whereas the 1988 compilation gives an ionization energy of 5.17 eV for the DMA isomer. Again, the latter value is calculated from heats of formation of the ion and the neutral. Recent mass spectrometric results from Castleman’s group9 have bracketed the appearance potentials (AP) of CH4N+ and C2H6N+ by looking at reactions of metal ions with suitable alkylamines. The values they obtained were 9.9 ( 0.1 eV e AP(CH4N+) e 10.6 ( 0.1 eV and 9.2 ( 0.2 eV e AP(C2H6N+) e 9.9 ( 0.1 eV. It was hypothesized that the ions in the two cases were MMA+ and DMA+. Most of the values cited in the above-mentioned compilations are derived either directly or Via some more circuitous route, from appearance potentials of ions resulting from fragmenta* To whom correspondence should be addressed. † Ohio State University Postdoctoral Fellow. Address as of 1st October 1995: Chemistry Department, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4408$12.00/0
tions. Obviously, appearance potentials should be treated with considerable caution, since they may differ from the ionization energy by a bond energy and/or internal excitation of the ion or an associated neutral fragment; some of the problems have been described by Chupka10 and Berkowitz.11 Another major problem is isomerization, for example, CH4N+, as noted above, could be CH2NH2+ or CH3NH+ and C2H6N+ could be C2H5NH+ or CH3NCH3+; other isomers are also possible (Vide infra). In addition to the presence of isomers, there is also the potential for complete misassignment of the species. As an example, if an ion of mass 30 appears from 1,2-dimethylhydrazine, this may be interpreted as CH4N+ or N2H2+. Then either isotopic substitution or higher resolution mass spectroscopy must be employed to distinguish between these possibilities. II. Historical Overview A. Experimental Results. (i) MMA. Collin12 measured the appearance potential of an ion of mass 30 resulting from the electron bombardment of various amines. It would seem likely that this was CH4N+. The appearance potentials varied from 10.0 to 11.7 eV, with primary amines giving the lower values. In similar experiments, Dibeler et al.13,14 measured two appearance potentials for an ion of mass 30: 11.0 and 13.0 eV. The ion was not definitively identified, with N2H2+ and CH3NH+ being offered. However, it was concluded14 that the ion was not C2H6+. Gallegos and Kiser15 looked at the electron impact spectra of some four- and five-membered rings and saw ions at mass 30. In two cases the ion was identified as CH4N+ (appearance potentials of 12.3 and 12.7 eV), and in one case it was identified as C2H6+. Fisher and Heath16 noted that electron impact studies (quoting unpublished data from M. J. Henchman) on deuterated methylamine had shown that the major hydrogen loss mechanism was to form CH2NH2+ and not CH3NH+. They derived an ionization energy of 6.25 eV for the CH2NH2 radical using a thermodynamic cycle. Collin and Franskin17 studied the electron impact mass spectra of methylamine, and from the mass spectra they deduced ionization energies of 6.05 and 7.49 eV for CH2NH2 and CH3NH, respectively. Louden and Webb18 hypothesized that the ionization energy of CH3NH was between those of NH2 and CH3NCH3. Griller and Lossing19 obtained an ionization energy of CH2NH2 of 6.2 eVsdeduced from the difference in heats of formation of the cation and the radical. A © 1996 American Chemical Society
IEs of Methylamidogens later study by Burkey et al. obtained a value of 6.1 eV.20 As noted above, Castleman and co-workers19 obtained upper and lower bounds for the ionization energy of CH3NH+ of 10.6 and 9.9 eV, respectively. Dyke et al.21 studied the CH2NH2 radical (produced from the reaction of F atoms with methylamine) using UV photoelectron spectroscopy. They deduced an adiabatic ionization energy of 6.29 ( 0.03 eV and a vertical ionization energy of 6.97 ( 0.03 eV. Ab initio calculations21 (Vide infra) indicated that CH3NH could not be giving rise to the observed photoelectron bands. (ii) DMA. Collin12 obtained appearance potentials of an ion of mass 44 that ranged from 9.4 to 13.0 eV. Dibeler et al.13,14 saw ions of mass 44, which they assigned either to the CH4NH2+ ion or to CH3NCH3+. In the latter cases, the appearance potential ranged from 9.7 to 11.1 eV. Gowenlock et al.22 calculated heats of formation of CH3NCH3+ and its neutral, allowing an ionization energy of ca. 9.1 eV to be calculated. Fisher and Heath16 deduced an ionization energy of 9.45 eV by considering a thermodynamic cycle. Later, Fisher and Henderson23 looked at mass spectra from a number of compounds and deduced the ionization energy of CH3NCH3 to be e9.42 ( 0.1 eV. As noted above, Castleman and co-workers9 obtain bounds on the ionization energy of DMA of 9.2 and 9.9 eV. It is perhaps worth adding that there are a number of structural isomers with the empirical formula C2H6N+. B. Ab Initio Calculations. (i) MMA. Lathan et al.,24 in an extensive study of first-row compounds, used STO-3G and 4-31G basis sets to calculate the geometry and relative energies of CH2NH2+ and CH3NH+. They calculated that CH2NH2+ was the more stable isomer. The ground state of CH3NH+ was calculated to be a 3A′′ state in the staggered (anti) conformation. MNDO calculations by Ford and Scribner25 also indicate the triplet state of CH3NH+ to be the ground state (although arylamidogen cations were shown to have singlet ground states). Bursey et al.26 used MINDO/3 calculations to study CH4N+ isomers. The lowest structure for CH3NH+ was the staggered conformation (the electronic state was not noted therein and was possibly a singlet state). Leroy et al.27 employed a 4-31G basis set at the UHF level to a number of neutral nitrogencentered radicals, and they used their results to calculate a number of thermodynamic quantites. Ford and Herman,28 in probably the most detailed calculation to date, used UHF/631G(d) and UHF/6-31G(d,p) calculations to optimize structures of amidogenic cations. They then performed higher level (UMPn, n ) 2-4) calculations at the same structure to obtain the energetics. Again, they found that the staggered 3A′′ structure was the most stable form of CH3NH. Attina et al.29 used the 3-21G basis set to optimize the geometry of CH3NH+, but only the singlet states appear to have been considered. Dyke and co-workers,21 as part of their photoelectron studies on the products of the F + CH3NH2 reaction, calculated the minimum geometry (ROHF/TZVP) of the 2A′′ state of the neutral as well as of the 3A′′ state of the cation. Vibrational frequencies were also reported in that work. Both of the states demonstrated a minimum in the staggered conformation. Vertical ionization ˜ 3A′′ a˜1A′) r energies were calculated for the CH3NH+ (X ˜ 2A′′) ionizations, with values of 10.16 and 10.45 CH3NH (X eV obtained at the ∆SCF + CI + Q level of theory (Q ) inclusion of the Davidson correction for quadruple excitations). This energy region of the photoelectron spectrum was obscured by other more intense features, and so the CH3NH radical was not conclusively observed in this work. Falvey and Cramer30 used UHF/6-31G* and AM1 CI calculations to optimize the structure of CH3NH+, and then CI calculations were carried out at the UHF/6-31G* geometries to compare calculated
J. Phys. Chem., Vol. 100, No. 11, 1996 4409 singlet-triplet gaps to semiempirical AM1 and MNDO calculations. One general conclusion was that the alkyl-substituted amidogens tended to have triplet ground states, whereas the arylsubstituted amidogens had singlet ground states. (ii) DMA. Jordan31 used STO-3G and 4-31G basis sets at the HF level of theory to calculate the minimum energy geometry of a number of C2H6N+ isomers. It is not clear from this paper what electronic state has been calculated, but it would seem to be a closed-shell singlet. The optimized configuration was for both methyl groups to be staggered to each other (in an anti-syn arrangement; the anti-anti configuration does not appear to have been considered). Ford and Scribner,25 in their MNDO study, calculate triplet cationic states to be more stable than the singlet states. Bursey et al.26 used MINDO/3 calculations to determine the stable isomers of DMA+, and their conclusion was that an eclipsed conformation was of lower energy than the staggered one, but it is not clear whether this was the syn-syn or anti-anti conformation. Barone et al.32 studied seven isomers of C2H6N+. Again, no indication of what electronic state is being optimized is given, but it appears to be a closed-shell singlet. Interestingly, in the latter paper, the totally eclipsed (syn-syn) conformation is calculated to be a minimum at the HF/4-31G and HF/6-31G*//HF/4-31G levels of theory, but by only 3 kJ mol-1 (0.7 kcal mol-1). (Note that, again, the anti-anti isomer does not seem to have been considered.) Falvey and Cramer30 calculate the geometry of the CH3NCH3+ isomer for both a triplet and a singlet state. Only partial geometric data are available in that work, and it appears that at the higher levels of theory employed therein, the triplet state is the ground state by 22.2 kcal mol-1. Ford and Herman,28 in 1989, calculated the energies of DMA cations using UHF/6-31G(d) theory. They showed that the 3B1 state with an anti-anti geometry was the global minimum. Very recently, Huang et al.33 studied the neutral CH3NCH3 radical at the UMP2/6-31G** level of theory. The lowest energy rotamer was also the anti-anti eclipsed conformation, which is in line with the conclusions of Cremer et al.,34 who note that the eclipsed, anti-anti conformation is the lowest energy form in general for dimethyl-X compounds, where X is a π-electron donor, owing to the formation of a pseudo-π bond between the two eclipsed pairs of hydrogens. III. Theoretical Methods The aim of this set of ab initio calculations was to calculate the adiabatic ionization energies (AIEs) of NH2, MMA, and DMA using the Gaussian-1 (G1) and Gaussian-2 (G2) methodologies as formulated by Pople and co-workers.35 The G2 theory, in particular, has been shown to yield very good values for various thermodynamic quantities, and in the case of ionization energies, an accuracy of (0.1 eV is usually achieved, making the method of “chemical accuracy” in this case. Although the methodologies have been shown to be reliable in many cases, in some cases they fail,36 and so some caution should be exerted. For this reason, it was decided to also calculate the ionization energy of the amidogen radical at a reasonably high level of theory, and then to use these calculations to calibrate the G1 and G2 theories and, if appropriate, to scale the AIEs of MMA and DMA, calculated with the G1 and G2 theories. To this end, the AIE of NH2 was calculated at a number of levels of theory, up to CCSD(T)/TZ2PF, using a restricted open-shell wave function in all cases. All calculations were performed with the ACES2 package.37 The ground electronic configuration of NH2 is 1a122a121b223a121b11, giving rise to a 2B1 ground state. The first excited state is formed by a b1 r a1 excitation, giving rise to a 2A1
4410 J. Phys. Chem., Vol. 100, No. 11, 1996
Wright and Miller
TABLE 1: Geometries, Frequencies, and Energies for NH2 and NH2+ at Various Levels of Theory, Calculated with the ACES2 Package state X ˜ 2B1 NH2
X ˜ 3B1 NH2+
r Å
θ degrees
ν1 cm-1
ν2 cm-1
ν3 cm-1
-(E + 55) hartree
1.013 1.028 1.020 1.023 1.025 1.025 1.024(5) 1.023 1.034 1.026 1.030 1.032 1.032 1.0338c
104.7 102.5 103.1 102.9 102.6 102.7 103.3(3) 143.0 148.1 149.3 150.3 150.5 152.0 153.17c
3633 3468 3471 3419 3388 3383 3219 3475 3356 3320 3281 3264 a 2900(50)
1644 1564 1573 1582 1573 1556 1497 1103 870 855 839 831 a 840(50)
3732 3585 3570 3504 3475 3475 3301 3714 3608 3570 3520 3504 a 3359.9
0.573263 0.737 038 0.775 146 0.784 386 0.798 063 0.814 131 0.192 451 0.342 722 0.372 808 0.392 662 0.395 510 0.411 213
level of theory HF/DZP MP2/DZP MP2/TZ2P CCSD/TZ2P CCSD(T)/TZ2P CCSD(T)/TZ2PF EXPTb HF/DZP MP2/DZP MP2/TZ2P CCSD/TZ2P CCSD(T)/TZ2P CCSD(T)/TZ2PF EXPTb
a These values could not be obtained, owing to technical difficulties. For the purposes of obtaining the ZPVE correction, the values at the CCSD(T)/TZ2P level were used for the ion. This is justified, since it was noticed for the neutral, the difference in calculated vibrational energies is very small between the CCSD(T)/TZ2P and CCSD(T)/TZ2PF levels of theory. b Experimental vibrational frequencies from Jacox, M. E. J. Phys. Chem. Ref. Data, Monogr. 3 1994, (computer version (VEEL3)). The experimental geometry for NH2 is from Dressler, K.; Ramsey, D. A. Philos. Trans. R. Soc. London 1959, 251A, 553. c There appears to be no reliable experimental value (see Okumura, M.; Rehfuss, B. D.; Dinelli, B. M.; Bawendi, M. G.; Oka, T. J. Chem. Phys. 1989, 90, 5918.), but probably reasonably reliable values are given by the ab initio calculation of Jensen, P.; Bunker, P. R.; McLean, V. A. D. Chem. Phys. Lett. 1987, 141, 53, which is quoted above.
state. The ground state of the ion is formed by removing an electron from the 3a1 orbital, giving rise to a 3B1 ground state as well as the corresponding open-shell singlet state.38 Both G1 and G2 methods are essentially approximations for calculating the thermodynamic quantities at the QCISD(T)/6311G**(2df) level for a molecular species by using single-point energy calculations at the UMP2(FU)/6-31G* optimized geometry and scaled UHF/6-31G* vibrational frequencies. In brief, energy corrections to the UMP4/6-31G*//UMP2(FU)/6-31G* energy are made in a systematic way in order to obtain the G1 energy; zero-point vibrational energy (ZPVE) is accounted for by scaling the frequencies calculated at the UHF/6-31G* level by a factor of 0.8929. The G2 method is merely a refinement of the G1 method, and the additional corrections are very simple to make. The G1 energy may be expressed as
EG1 ) Ee + ZPVE
(1)
Ee ) E(combined) + ∆E(HLC)
(2)
E(combined) ) E(UMP4/6-311G**) + ∆E(+) + ∆E(2df) + ∆E(QCI) (3) (The nomenclature of the original references35 has been used.)
EG2 ) EG1 + ∆ + ∆(HLC)
(4)
where ∆ is an additional correction to the total energy and ∆(HLC) is a correction to ∆E(HLC). All the details of the calculations that need to be performed are clearly described in the original references.35 Gaussian 9239 was used for all G1 and G2 calculations. For CH3NH, the staggered (anti) conformation was used (2A′′ electronic state, Cs symmetry), while for (CH3)2N, the eclipsed anti-anti configuration was used (2B1 electronic state, C2V symmetry). For the cations, the triplet state was used [MMA (3A′′ electronic state, Cs symmetry), DMA (3B1 electronic state, C2V symmetry)]. Since, in G1 and G2 theory, the unrestricted Hartree-Fock methodology is employed, it is necessary to check that spin-contamination is not a problem (or the calculated geometries and frequencies may be unreliable). In fact for the neutral doublet states, 〈S2〉 was about 0.76, and for the cationic
TABLE 2: Calculated Adiabatic Ionization Energies for NH2 at Various Levels of Theory
a
level of theory
adiabatic ionization energy/eVa
HF/DZP MP2/DZP MP2/TZ2P CCSD/TZ2P CCSD(T)/TZ2P CCSD(T)/TZ2PF G1 G2 EXPT
10.32 10.68 10.90 10.61 10.90 10.91 10.99 11.01 11.14 ( 0.01
Including ZPVE corrections.
triplet states 〈S2〉 was less than 2.05, implying that spincontamination is of minimal importance in these systems. For the energetics, the total energy after projection was used for all UMPn calculations. IV. Results The calculated geometries and vibrational frequencies for the amidogen radical and its cation are presented in Table 1 from the calculation with the ACES2 package. As may be seen, successively higher levels of theory give improvements in the calculated values when compared to experiment. In particular, the geometries are in excellent agreement. The vibrational frequencies are also in good agreement, remembering that harmonic values are calculated, whereas experiment generally gives vibrational separations. This being the case, it is expected that the calculated ionization energies would also be fairly good. However, it is generally true that calculated energy separations are more unreliable than molecular geometries and vibrational frequencies. The calculated ionization energies are given in Table 2. As may be seen, even at the highest level of theory used (CCSD(T)/TZ2PF), there is still a significant error in the calculated ionization energy (of 0.2 eV) compared to the quoted experimental error of 0.01 eV. Next, the G1 and G2 methodologies were followed to calculate the adiabatic ionization energy of the amidogen radical. The relevant energies are given in Table 3, and the different corrections are explained in detail in the original references that describe the G1 and G2 theories. The calculated ionization energies at the G1 and G2 levels of theory are given in Table
IEs of Methylamidogens
J. Phys. Chem., Vol. 100, No. 11, 1996 4411
TABLE 3: Energies and Energy Correctionsa for NH2, MMA, and DMA and Their Cations from the G1 and G2 Calculations -MP4/MP2b
∆E(+)
∆E(2df)
∆E(QCI)
∆E(HLC)
-Ee
-EG1
∆
∆(HLC)
-EG2
species
55.309 81 55.753 10 94.613 53 94.962 24 133.855 22 134.173 30
-0.23 -4.86 -1.09 -5.11 -2.02 -5.66
-17.89 -26.61 -39.88 -46.81 -61.04 -67.69
+0.06 -1.34 -1.86 -0.91 -2.34 -1.27
-18.80 -18.61 -43.36 -49.31 -67.92 -73.87
55.379 83 55.804 52 94.607 69 94.965 11 133.988 53 134.321 79
55.382 38 55.786 16 94.562 60 94.917 88 133.915 05 134.247 15
-4.40 -6.28 -10.15 -11.02 -14.88 -15.26
2.28 3.42 7.89 9.12 12.54 13.68
55.384 50 55.789 02 94.564 77 94.919 78 133.917 39 134.248 73
NH2+ NH2 CH3NH+ CH3NH (CH3)2N+ (CH3)2N
a
Energies in hartrees; corrections in millihartrees. b MP4/6-311G**//MP2(FU)/6-31G*.
TABLE 4: Calculated Ionization Energies (eV) for MMA and DMA to the Ground Triplet State of the Cation species
G1
G1 (scaled)
G2
G2 (scaled)
CH3NH (CH3)2N
9.67 9.04
9.80 9.16
9.66 9.02
9.78 9.13
2 and are seen to be almost identical. These two values are in good agreement with the value obtained at the CCSD(T)/TZ2PF level of theory (see Table 2)sand while there remains a small, but still significant, disagreement with the experimental value (although the error (∼0.1 eV) is about that expected from G1 and G2 theory), the discrepancy with the experimental value using the G2 calculation is only about half that obtained using the highest level of theory [CCSD(T)/TZ2PF] of the ACES2 package. It may be seen that the G2 value is 0.988 times that of the experimental value, while the corresponding G1 value is 0.9865. Under the assumption that the experimental value is as accurate as it is precise, it was decided that reasonably accurate values for the ionization energies of CH3NH and (CH3)2N could be obtained by calculating their ionization energies at the G1 and G2 levels of theory and then scaling them by the same factor as in the NH2 case. This approach is clearly preferable to using the much more expensive CCSD(T)/TZ2PF calculations for MMA and DMA. For MMA and DMA, the calculated energy corrections for the G1 and G2 calculations are presented in Table 3, and the ionization energies derived from the G1 and G2 energies are given in Table 4. Similarly, Table 4 gives scaled ionization energies using the scaling factors from NH2. Thus, from these calculations, it may be inferred that the ionization energies of CH3NH and (CH3)2N are 9.8 ( 0.1 and 9.1 ( 0.1 eV, respectively, where the error limits are estimated from the results for NH2. V. Discussion First of all, with regard to the compilations mentioned in the Introduction, it may be seen that the lower of the values quoted in the 1977 Rosenstock et al. compilation7 are of a similar magnitude as the values derived here for both MMA and DMA (although the data were surmised to be for the CH2NH2 isomer therein). Thus, it may be reasonably assumed that these lower appearance potentials probably correspond to the ionization energy of the MMA radical; the higher values probably correspond to some fragmentation processes. On the other hand, the 1988 compilation of Lias et al.8 has a value of 6.7 eV for the CH3NH isomer. This is clearly in error and probably corresponds to CH2NH2, since the value lies between the adiabatic ionization energy (6.3 eV) and the vertical ionization energy (7.0 eV) as measured by photoelectron spectroscopy.21 With regard to the DMA isomer, the value of 5.17 eV quoted by Lias et al. is also clearly in error; it must correspond to some other isomer of C2H6N or it may just be incorrect, since heats of formation of the neutral and cation were used, and either or both of these may be incorrect. Further G2 calculations using reaction schemes to derive heats of formation of the neutral
and cationic states of all possible isomers would help to localize the possible errors, but they are outside the scope of this work. Considering now the experiments by Castleman and coworkers,9 where the ionization energy of the (assumed) MMA and DMA isomers were bracketed, it may be seen that the calculated value of 9.8 ( 0.1 eV is just at the low end of the deduced range of 9.9 ( 0.1 to 10.6 ( 0.1 eV. Similarly, for DMA, the calculated value of 9.1 ( 0.1 eV is at the low end of the range 9.2 ( 0.1 to 9.9 ( 0.1 eV. Although the calculated values are just within the experimental error range, it must be remembered that they were derived from a scaling of the calculated G1 and G2 values and that this scaling is probably only approximately correct. Thus, the calculated values may be said to be consistent with the lower range of values from Castleman and co-workers, and this adds some weight to both studies. The fact that the two sets of values differ so much from those in the compilation of Lias et al.8 makes it very unlikely that the latter is correct. The value derived here for DMA is also in good agreement with the results of much earlier determinations (Vide supra)sparticularly those of Gowenlock et al.22 (9.1 eV) and Fisher and Henderson23 (e9.42 ( 0.1 eV). For all three species, NH2, MMA, and DMA, the ionization may be thought of as a removal of a nonbonding lone-pair electron from the nitrogen. This explains the observed increase in bond angle from ca. 110° to ca. 150°, for NH2 upon ionization, which is the main geometric change upon ionization and gives rise to the progression in the bending vibration seen in its photoelectron spectrum.38 Additionally, as the series of compounds is transversed, with one and then two methyl groups being added to the nitrogen atom, the effect is to lower the ionization energy, as one would expect from simple organic chemistry arguments where the methyl group is thought of as an “electron pusher”, thus increasing the electron density on the nitrogen and lowering the ionization energy. Alternatively, the methyl groups may be thought of as stabilizing the cation by quenching the positive charge, either by induction or by mesomeric effects (Via hyperconjugation). One question that needs to be asked is what methods would lead to conclusive experimental determinations of the AIEs for MMA and DMA? Clearly, the radicals need to be generated so that there is no question of fragmentation. Several methods have been published for the possible generation of alkylamidogen radicals, and these include pyrolysis,40 photolysis,41 infrared multiphoton dissociation,42 and fluorine atom extraction.43 Once the radicals have been formed, they can be studied using standard mass spectrometric or photoelectron spectroscopy techniques. Of course, both techniques will suffer from loss of selectivity, since other radicals may also be formed in the radical production step. One efficient way around this would be to use spectroscopic selection Via multiphoton ionization. Then a photoionization efficiency curve or even a zero-kineticenergy (ZEKE) photoelectron spectrum could be recorded through an intermediate resonant state, thereby avoiding the difficulties of other radicals being present. One possible difficulty with all these techniques, for DMA in particular, is that, as Levsen and McLafferty have shown,44 the DMA cation
4412 J. Phys. Chem., Vol. 100, No. 11, 1996 isomerizes rather quickly (10-5 s). However, it is not certain how much internal energy the isomerizing cation had in that work; it may be that the cation is long-lived when it contains little internal energy. VI. Conclusion The ionization energy of the amidogen radical has been calculated at various levels of theory. It has been shown that fairly high levels of theory still show a small discrepancy with the experimentally derived value of the ionization energy. The computationally inexpensive G1 and G2 methods yield very similar ionization energies that are in fact slightly closer to the experimental value. G1 and G2 levels of theory were then used to calculate the ionization energies of the MMA and DMA species, and these were found to disagree to an enormous extent with those from the most recent compilation. However, the values were in close agreement with the values derived from a later experiment by Castleman and co-workers.9 Acknowledgment. The authors gratefully acknowledge the support of this work by the National Science Foundation via Grant CHE 9320909, by a grant of supercomputer time from the Ohio Supercomputer Center, and by the Ohio State University for a postdoctoral fellowship for T.G.W. References and Notes (1) Wieland, H. Ann. Chem. 1912, 392, 127; 1912, 392, 133. (2) Miller, J. A. Cancer Res. 1970, 30, 559. Kriek, E. Biochim. Biophys. Acta 1974, 355, 177. Miller, E. C. Cancer Res. 1978, 38, 1479. Miller, E. C.; Miller, J. A. Cancer 1981, 47, 2327. (3) See, for example, the following. Nelsen, S. F.; Landis, R. T.; Kiehle, L. H.; Leung, T. H. J. Am. Chem. Soc. 1972, 94, 1610. Handoo, K. L.; Cheng, J.-P.; Parker, V. D. Acta Chem. Scand. 1993, 47, 626. (4) Gassman, P. G. Acc. Chem. Res. 1970, 10, 26. Abromovitch, R. A.; Jayaraman, R. In Azides and Nitrenes: ReactiVity and Utility; Scriven, E. F. V., Ed.; Academic Press: Orlando, FL, 1984. (5) See, for example, the following. Radecki. B. D.; Allison, J. J. Am. Chem. Soc. 1984, 106, 946. (6) Gibson, S. T.; Greene, J. P.; Berkowitz, J. J. Chem. Phys. 1985, 83, 4319. (7) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys. Chem. Ref. Data, Suppl. 1 1977, 6. (8) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data, Suppl. 1 1988, 17. (9) Sigsworth, S. W.; Keesee, R. G.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1988, 110, 6682. (10) Chupka, W. A. J. Chem. Phys. 1959, 30, 191. (11) Berkowitz, J. Photoionization, Photoabsorption and Photoelectron Spectroscopy; Academic Press: New York, 1979. (12) Collin, J. Bull. Soc. Chim. Belg. 1953, 62, 411. (13) Dibeler, V. H.; Franklin, J. L.; Reese, R. M. J. Am. Chem. Soc. 1959, 81, 68. (14) Dibeler, V. H.; Franklin, J. L.; Reese, R. M. AdV. Mass Spectrom. 1959, 1, 443.
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