Carbon-13-proton coupling constants in carbocations. II. Angular dependence of 1JCH in groups adjacent to cationic carbons. A new criterion for interpreting NMR spectra of carbocations (original) (raw)
Related papers
Journal of the American Chemical Society, 1984
Figure 4. Plot of AJCiw- ,-arbony, in 0-protonated benzaldehydes vs. those in 0-protonated acetophenones. that there is a linear free energy relationship between AJ values of two similar system. A plot of AJci,-c,b,, values in substituted 0-protonated benzaldehydes against the corresponding values in 0-protonated acetophenones is also linear (Figure 4) (r = 1 .O). These linear plots of AJ values indicate that the substituent effect on Jcc values (SCC) in systems t h a t are closely related in their electron demand is very similar. Jcc values can, a s shown in the present study, be valuable in the study of carbocationic systems. W e are continuing our further study on 13C-13C coupling in such electron-deficient systems. Experimental Section All acetophenones and benzaldehydes used were commercially available samples with 299% purity. Regular 13C NMR and I3C satellite spectra were recorded at 50.3 MHz with use of a Varian XL-200 su-perconducting NMR spectrometer equipped with a variable-temperature broad-band probe. The spectra of neutral acetophenones and benzaldehydes were obtained at room temperature in CDCI, solvent (-30% solution). The 0-protonated carboxonium ions were prepared by slow addition of the corresponding acetophenone or benzaldehyde to FSO3H at ca.-40 ' C (-25% solution), and the spectra were recorded in FSO3H at-35 'C. The pulse sequence used for the I3C satellite spectra, based on Freeman et aI.,l7 is 90' (x)-+-180°Cy)-r900(x)-A-90'(~)-Acq.(+), where T N (2n + 1)/4Jcc, A is a very short delay (-10 f i s) needed to reset the radio-frequency phase during which time double quantum COherance evolves,and $J and + are the phase of the last 90' "read" pulse and the receiver reference phase, respectively. Optimum setting of T for direct coupling is when n = 0 and thus is set at 4.5 ms (corresponding to a Jcc value of-55 Hz). The repetition rate of this sequence is-10 s, and reasonable S/N was achieved in 6-8 h of acquisition. The coupling constants can be directly measured from the "satellite" spectra." The accuracy of the coupling constants in a neutral compound are 10.2 Hz while those in the protonated carboxonium ions are 1 0. 3 Hz. Some of the coupling constants could not be measured accurately due to overlap of peaks or due to broadening because of slow rotation around the Cipo-Ccsrbnyl bond (in the carboxonium ions).
Journal of The American Chemical Society, 1999
High level ab initio MO calculations have identi®ed two complexes of borane with 2-¯uoropropane. In the ®rst, the boron atom is above the small C1,C2,C3 angle (in complex); in the other, the boron atom is outside that angle (out complex). The ionization to form the 2-propyl cation and the trihydro¯uoroborate ion A was followed by lengthening the C±F bond, d, in each complex and reoptimizing all other geometrical parameters. It was concluded that ionization occurred at d 1:8±2:0 A: The out ionization pathway was of lower energy (1±2 kcal/mol at MP2(FC)/6-31G pp ) at al values of d. A third pathway (top), in which the F±B bond was held perpendicular to the C1,C2,C3 plane, rather than allowed to tilt, was 8±9 kcal/mol higher in energy throughout. Ionization to the tight ion pair did not require a large energy expenditure (19.0 kcal/mol over the isolated reactants at d 2:00 A; 30.3 kcal/mol at d 2:20 A). A medium of dielectric constant 8.93, in SCRF(IPCM)-MP2(FC)/6-31G pp yyMP2/6-31G pp calculations, reduced the ionization energy to 14.2 kcal/mol at d 2:20 A: The dielectric medium also reorders the relative energy of the three orientations to out , top (1.0 kcal/mol) ,in (4.5 kcal/mol), at d 2:20 A: Changes in bond lengths and angles show hyperconjugative assistance to the ionization by the methyl hydrogens anti to the¯uorine along all pathways. As the ions separate, this effect decreases and. the interaction with the anion of one of the hydrogens initially syn to the¯uorine becomes important. Along the in pathway, rotation of one of the methyl groups occurred along the in pathway at 2.20 A Ê and elimination at 2.30 A Ê ; along the top pathway, it occurred at 2.30 and 2.40 A Ê , respectively. Along the out pathway, methyl group rotation brought a hydrogen in each close to the anion at d 2:50 A and elimination followed at 2.70 A Ê . The top orientation had not eliminated at distances about 2.6 A Ê and higher in our previous work. Both the in and out forms were optimized without elimination at d 2:80 A with tetra¯uoroborate (C) as anion. The calculations predict features of solvolyses, such as transfer of a nucleophile from the backside of the anion (retentive solvolysis), elimination to ole®n within the ion pair, and recombination at the backside of the anion (oxygen scrambling faster than racemization of tosylates and carboxylates). q
Journal of The American Chemical Society, 1979
The cy-ethynylvinyl (4), n-ethenylvinyl (5), a-cyclopropylvinyl (6), and a-phenylvinyl (7) cations have been investigated by SCF-MO ab initio methods, using both the STO-3G and the 4-31G basis sets. The cations 5,6, and 7 are more stable in perpendicular conformations (5a, 6a, and 7a, respectively) where the interaction between the "empty" cationic orbital and the HOMO of the substituent is maximized. The calculated rotation barriers around the C+-substituent bonds are 22.2, 15.8, and 24.7 kcal/mol for 5,6, and 7, respectively, approximateiy half the barrier in the corresponding primary alkyl cations. The efficiency of the n substituent in stabilizing the vinyl cation follows the order CsH5 > c-CsH5 N HC=CHz >> C=CH = CH3 >> H. The ability of the substituents to donate electrons to the empty cationic orbital follows the order C6H5 > CH=CH* > C=CH > c -C~H~ > CH3 > H. No correlation is found between the total charge a t the cationic center or the corresponding populations of the formally empty p orbital and the stability of the cation. The cations vinyl (21, a-methylvinyl (3), and 6 have stabilities which are intermediate between those of the corresponding primary and secondary alkyl cations. However, the r-stabilized cations, 4,5, and 7 are of comparable stability to the corresponding primary alkyl cations. Corresponding substituted ethyl cations are 12-17 kcal/mol more stable than the vinyl cations, suggesting that, for the groups examined here, substituent effects are inherently similar for alkenyl and for alkyl cations. The proton affinities of substituted acetylenes and olefins are comparable, with the olefins being 1-5 kcal/mol more basic.
The Journal of Organic Chemistry, 2004
Cyclohexane (1), oxygen-, sulfur-, and/or nitrogen-containing six-membered heterocycles 2-5, cyclohexanone (6), and cyclohexanone derivatives 7-16 were studied theoretically [B3LYP/6-31G(d,p) and PP/IGLO-III//B3LYP/6-31G(d,p) methods] to determine the structural (in particular C-H bond distances) and spectroscopic (specifically, one bond 1 JC-H NMR coupling constants) consequences of stereoelectronic hyperconjugative effects. The results confirm the importance of nX f σ*C-Ηapp (where X) O, N), σC-Hax f π*CdO, σS-C f σ*C-Ηapp, σC-Sfσ*C-Ηapp,-nO f σ*C-H, and σC-H f σ*C-Ηapp hyperconjugation, as advanced in previous theoretical models. Calculated rC-H bond lengths and 1 JC-H coupling constants for C-H bonds participating in more than one hyperconjugative interaction show additivity of the effects.
Molecular mechanics (MM3) calculations on aldehydes and ketones
Journal of the American Chemical Society, 1991
the hyperfine coupling constants measured in derivatives of C O Tmay be providing less information about the degree of bond alternation at the equilibrium geometries of these radical ions than about the ease with which geometries with more nearly equal bond lengths are accessed. On the basis of INDO calculations, Hammons, Bernstein, and MyersI3 have advanced explanations of the effects of substituents on the EPR spectra of COT'that are similar to those presented here. Other r e~e a r c h e r s~~.~~ have proposed an alternative model, which assumes bond alternation does not occur, so that the NBMOs in eq 1 are not mixed. Instead, this latter model postulates that there is a Boltzmann poDulation of the lowest excited methyl, and cyano derivatives. Our calculations suggest that these substituents have a relatively small effect on the extent of bond length alternation at the equilibrium geometry. However, both F, a A donor, and CN, a A acceptor, are found to reduce substantially the barrier to bond equalization. Acknowledgment. We thank the National Science Foundation for its support of this research and for providing funds that enabled the purchase of the Convex C-2 computer, on which some of the calculations reported here were performed. We also thank the San Diego Supercomputer Center for a generous allocation of time on the Cray YMP-8/864 computer at SDSC. electronic state in which one eledtron is thermally excited from Registry No. C O T , 34510-85-5; F-COT, 70741-95-6; CH3-COT, the lower energy of the two NBMOs to the upper. 3451 9-36-3; CN-COT, 70741-98-9. Our CI calculations indicate that the basic assumption of the latter model is incorrect, since we find that bond alternation is energetically favorable, not only in COT*-, but also in its fluoro, Supplementary Material Available: ROHF/3-21G-optimized geometries and ROHF and CI energies for bond-alternated (C h angle-alternated (C,,,), and midpoint geometries of fluorocvclo- .-,ocktetraene radical anion (2 pap&). OTdering information is given on any current masthead page.
Journal of the American Society for Mass Spectrometry, 1996
Metastable ion decompositions, collision-activated dissociation (CAD), and neutralization-reionization mass spectrometry are utilized to study the unimolecular chemistry of distonic ion 'CH 2CH 2CH-OH (2 +,) and its enol-keto tautomers CH 3CH=CHOH-' (1+') and CH 3CH 2CH=0+' (3+"). The major fragmentation of metastable 1+'-3+' is H' loss to yield the propanoyl cation, CH 3CH 2C=0+. This reaction remains dominant upon collisional activa tion, although now some isomeric CH 2 = CH-CH +OH is coproduced from all three precursors. The CAD and neutralization-reionization (+NR+) spectra of keto ion 3 +. are substantially different from those of tautomers 2 +. and 1 +'. Hence, 3 +. without sufficient energy for decomposition (i.e., "stable" 3 +,) does not isomerize to the thermodynamically more stable ions 2 +, or 1 +', and the 1,4-H rearrangement H-CH 2CH 2CII=0+'(3+')-> 'CH 2CH 2CH+0-H (2+') must require an appreciable critical energy. Although the fragment ion abundances in the +NR + (and CAD) spectra of 1 +. and 2 +. are similar, the relative and absolute intensities of the survivor ions (recovered C 3H 60+' ions in the +NR + spectra) are markedly distinct and independent of the internal energy of 1 +. and 2 +'. Furthermore, 1 +. and 2 +' show different MI spectra. Based on these data, distonic ion 2 +. does not spontaneously rearrange to enol ion 1 +, (which is the most stable C 3H 60+' of CCCO connectivity) and, therefore, is separated from it by an appreciable barrier. In contrast, the molecular ions of cyclopropanol (4 +.) and allyl alcohol (5 +.) isomerize readily to 2 +', via ring opening and L 2-H-shift, respectively. The sample found to generate the purest 2 +. is a-hydroxy-y-butyrolactone. Several other precursors that would yield 2 +. by a least-motion reaction cogenerate detectable quantities of enol ion 1 +', or the enol ion of acetone (CH 2 =C(CH 3)OH+', 6 +'), or methyl vinyl ether ion (CH 30CH=CH;-, 7+'). Ion 6+' is coproduced from samples that contain the-CH 2-CH(OH)-CH 2-substructure, whereas 7 +. is coproduced from compounds with methoxy substituents. Compared to CAD, metastable ion characteristics combined with neutralization-reionization allow for a superior differentiation of the ions studied. UAm Sac Mass Spectrom 1996, 7, 573-589) R ad ical cations C 3H 60+' with the CCCO frame appear in the mass spectra of many organic compounds [1] and, therefore, have been investigated extensively both theoretically [2-6] and experimentally [7-14]. It is now established that enol ion CH 3CH=CHOH+' (1 +.) is the most stable isomer of CCCO connectivity [15] and that the threshold dissociation of this cation yields the propanoyl ion,