Carbon-13 magnetic resonance shifts were determined for six families of 1,4-disubstituted benzenes. Linear least-squares correlations between the internal shifts of the families suggest an additivity relationship for 13C substituent effects at all four nonidentical ring carbons and at the a position when one of the substituents has a central carbon. The correlation of and 18C shifts for these compounds is also linear, implying that the additivity relationships derived in a previous study can be generalized to 13C.
The charge-energy relationships in Table IV were obtained using atomic charges computed with molecular wave functions of near-Hartree-Fock accuracy.3•4 (33) Two procedures for estimating atomic charges from experimentally determined, rather than calculated, inner shell energies have recently been proposed.34•35 The charges obtained are based upon other definitions of atomic charge than that used in the present work, which has as its basis the molecular electronic density function.(34) D.
, 554 (1984). Electron impact (EI) mass spectral studies of methyl diazoacetate, ethyl diazoacetate (I), methyl diazoacetoacetate (2). and dimethyl diazomalonate (3) indicate that loss of N2 is exceedingly difficult from the parent ion and was observed only with 3. Diazoacetylacetone exhibits an appreciable (M -28)'. Exact Mass and metastable transitions indicate fragmentation occurs in part via 1,2,3-oxadiazolium ion-radicals (6) or a-lactones (7) There are surprisingly few studies of the mass spectra of diazo compounds (2-12). Frequently the generation of the ion [RCN,]' occurs (presumably the diaza analog of the cyclopropenium ion) (2, 3, 5, 6, 1 1, 12). The reported spectrum of diazomethane (2) exhibited this as a peak of 8.4% intensity and interestingly, possesses an M+ of 96.5%. Paulett and Ettinger (2) also found a value for D(CH2 -H2) of 1.9 eV and for D(CH2 -N2)+, 3.3 eV.Our observation (10) that dimethyl diazomalonate failed to lose N2 to form a (M -28)' ion is somewhat unique since the M+ ion might be expected to readily lose N2 to furnish a carbene-cation-radical stabilized by its substituents. This would have mimicked the reported photochemical behavior (13), a common occurrence in photochemistry and mass spectroscopy (14). The lack of mimicry here is especially surprising since the n system is fully conjugated. Ions possessing the necessary lifetime to pass through a mass spectrometer might be expected to have sufficient time to undergo electronic decay to the most stable cation radicals regardless of which electron had been ejected by electron bombardment. We have now completed a study of the series of structurally related diazo carbonyl compounds, ethyl diazoacetate (I), methyl diazoacetoacetate (2), dimethyl diazomalonate (3), and 3-diazopentane-2,4-dione (4). Their low and medium-high resolution EI mass spectra are tabulated in Tables 1-5. Spectra were obtained at -75, -30, and -13 eV. The experiments were performed using a JEOL-Dl00 double focussing mass spectrometer equipped with a JEOL data system and a JEOL ion defocussing attachment capable of scanning accelerating voltage and thus observing metastable transitions (15). The '~uthor to whom correspondence may be addressed.Undergraduate research participant.
IRevision received October 14, 1983.D-100 can be operated in either a low resolution mode or a medium-high resolution mode (up to -10 000 resolution). For the mass ranges and compositions encountered here, it was possible to distinguish between the various elemental compositions for the ions of interest. The inlet system is all glass up to the gap before the ionizing source. Thus, this study was free from the plague of metal catalyzed processes to be expected of diazo compounds and observed in some of the earlier studies in this area (3). Table 6 reveals some of the common fragmentations observed. Table 7 is a listing of all of the nitrogen-containing fragments found in the m s of methyl diazoacetoacetate. Schemes 1-3 reveal the fragmentation sequences obtained by ion defocussing.All ...
Evidence of first-order alkyl-ring proton coupling is reported for the 4-alkylnitrobenzenes. Typical coupling constants of 0.7 Hz and 0.3 Hz were found for α-alkyl coupling with the ortho and meta ring protons, respectively, when the alkyl function was methyl, ethyl, and isopropyl. No β-alkyl coupling with ring protons was observed for the ethyl, isopropyl, or t-butyl groups, suggesting that the magnitude of the coupling constant is vanishingly small.
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