Gas‐phase chemistry of the methyl carbamate radical cation, H₂NCOOCH. II—A test case for ion structure assignment

Abstract

The unimolecular chemistry of the methyl carbamate radical cation, H2NCOOCH 3+·, 1, has been further investigated by a combination of mass spectrometry‐based experiments (metastable ion (MI), collisional activation (CA), collision‐induced dissociative ionization (CIDI), neutralization‐reionization (NR) Spectrometry and 2H labelling) and ab initio molecular orbital calculations, executed at the MP3/6–31G*//4–31G level of theory and corrected for zero‐point vibrational energies. Apart from the previously located maxima, i.e. H2NCOOCH3+·, 1, the distonic ion H2NC(OH)OCH3+·, 2, hydrogen‐bridged ions [H2NCO…︁ H…︁OCH2]+·, 5, and [H2NCHO…︁…︁H…︁OCH]+·, 7, there exist at least two other equilibrium structures, viz. the iminol ion HNC(OH)OCH 3+·, la, and the hydrogen‐bridged species [H2CO…︁H…︁N(H)COH] 3+·, 6a, which is closely related to ion 5. Although the iminol ion la lies only 30 kJ mol−1 above 1, our calculations indicate that the barriers for its formation either directly from ionized methyl carbamate 1 via a 1,3‐hydrogen shift or indirectly via 1,4‐hydrogen shifts from the distonic ion 2 are too high to allow the iminol ion to be involved in the unimolecular chemistry of ionized methyl carbamate. This explains the earlier observation that there are no H‐D exchange reactions prior to decomposition of ionized labelled methyl carbamate, in contrast to the related ion methyl acetate. However, attempts to generate the iminol ion by loss of CH3CN from CH3CHNNHCOOCH3 produced the more stable distonic ion 2 instead, but it proved very difficult to assign its structure unequivocally because 2 can rapidly interconvert with 1 and so virtually identical dissociation characteristics ensue. Only by integration of results obtained from many experiments and from ab initio calculations could structure 2 be assigned. The distonic ion 2 can undergo two transformations: after stretching of the COCH2 bond the incipient formaldehyde can migrate within the electrostatic field of ionized hydroxyaminocarbene to the OH end to generate 5, but it can also migrate to the NH end to generate 6a. This explains the previous puzzling observation that H2NCOOCD 3+· forms both CD2OD· and CD2OH· in CA and NR experiments. The calculations and experiments indicate that, although the ion is exceedingly difficult to characterize, the distonic ion 2 is the key intermediate for all the observed dissociations of methyl carbamate.

The Structure of 1H‐Perimidin‐2(3H)‐one and Its Derivatives in the solid state (x‐ray crystallography and CP/MAS 13C‐NMR), in solution (13C‐NMR), and in the gas phase (mass spectrometry)

“…The H,O loss could occur through a [1,3]-H transfer (from N to 0) which is energy costly (ca. 43 kcal mol-' [25]). On the other hand, AM1 calculations [I91 [26] show that 2bf' ions are more stable than 2a+' ions by 5.0 kcal mol-I (193.5 and 198.5 kcal mol-', resp.).…”

Catalysis in Hydrogen-Bridged Radical Cations

“…Proton-transport catalysis as a key step in the dissociation of organic ions PTC not only occurs under certain conditions in bimolecular reactions (see above), it is also a key step in the dissociative rearrangement of a variety of oxygen-containing [30][31][32][33][34][35][36][37] and nitrogen-containing [38][39][40][41][42] radical cations. The dissociation chemistry of ionized ethylene glycol 31 represents an early example of this point.…”

“…The final steps of the reaction involve charge transfer, followed by rotation of the incipient formaldehyde ion into ion EG-5, the reacting configuration for the loss of HCO • . Other vicinal diols, 31 b-ketols 30 and their analogues [37][38][39] display the same basic catalysis pathway.…”

Differentiation of diastereomeric conduramine derivatives under electron ionization and chemical ionization mass spectral conditions

“…The EI mass spectra of 3 and 4 show the [M-CH 3 ] þ ion at m/z 270, but not a M þ. ion ( Table 1). The fragmentation pattern for (7), 124 (9), 122 (12), 116 (15), 114 (6), 111 (15), 110 (21), 109 (17), 108 (16), 101 (7), 100 (9), 88 (13), 85 (20), 81 (82), 80 (15), 68 (7), 59 (22), 43 (81)…”

“…Since the CID spectrum of the [M-CH 3 COOH] þ ion (m/z 225) did not show loss of CO, it is reasonable to explain the formation of m/z 197 from M þ. by initial loss of CO followed by that of CH 3 COOH, but not vice versa. Further, the loss of CO must be from the carbamate group (-NHCOOCH 3 ) as methyl carbamate is known to lose CO. 16,17 Another typical fragment ion at m/z 167 corresponds to the sequential loss of CH 3 COOH and C 3 H 6 O, or vice versa, from M þ. ; it appeared at the same m/z value in the corresponding deuterated compounds (d 3 and d 6 ) thus confirming the involvement of both the acetyl and the dioxalane group in its formation. The studied isomeric pairs (1-4) are diastereomers and differ only in the stereochemistry of the substituent (-OR, where R --H or COCH 3 ) at position 3 (Scheme 1), and for convenience we denote these isomers as cis-(1 and 3) and trans-isomers (2 and 4).…”