Calculated ¹¹ B– ¹³ C NMR chemical shift relationship in hypercoordinate methonium and boronium ions

Abstract

The boronium-carbonium continuum was extended to include hypercoordinated protonated methanes and their boron analogs. RESULTS AND DISCUSSIONThe structures of the hypercoordinate protonated methanes and their boron analogs have been optimized at the MP2͞6-31G** level by using the (8) package of programs. 13 C and 11 B NMR chemical shifts were calculated by the correlated GIAO-MP2 method (9, 10) (tzp͞dz basis set; refs. 9 and 10) using the ACES II programs (11). CH 5 ؉ and BH 5 . CH 5 ϩ (protonated methane) is considered the parent of nonclassical carbocations containing a fivecoordinate carbon atom. The C s symmetrical structure 1a with a three-center, two-electron (3c-2e) bond is the preferred ground-state structure for the CH 5 ϩ (12-16). The isoelectronic boron analog of CH 5 ϩ is neutral BH 5 and is also C s symmetrical 1b based on high-level ab initio calculations (17, 18).The 13 C and 11 B NMR chemical shifts of 1a and 1b were calculated by using the GIAO-MP2 method (9, 10) ( Table 1) C of methane is Ϫ2.3. The shielding effect in 1a, however, is expected for such a hypercoordinate (nonclassical) carbocation (6, 7). Evidence for CH 5 ϩ in the condensed phase came from hydrogen-deuterium exchange on methane in deuteriated superacids (21,22) and ESCA studies (G.A.O., unpublished results). Direct NMR spectroscopic observation of the ion was not yet achieved and is difficult because of its expected low concentration and fast exchange in superacids.The calculated ␦ 11B of 1b is Ϫ28.8. We also estimated the ␦ 11 B values in 1b by applying Eq. 2 and by using the ␦ 13 C value of 1a. This yielded a ␦ 11

Theoretical Investigations on the Formation and Dehydrogenation Reaction Pathways of H(NH2BH2)nH (n = 1−4) Oligomers: Importance of Dihydrogen Interactions

“…The stability of the dihydrogen interaction with B is not unexpected given that the B center is electron-deficient such that formation of a σ-complex with a R−BH 2 (η 2 -H 2 ) moiety is energetically feasible without violating the classic octet rule . Indeed stable BH 3 (η 2 -H 2 ) molecules and BNH x dihydrogen intermediates are found in our calculations and also in previous research on boron systems. , …”

Relativistic DFT calculations of the NMR properties and reactivity of transition metal methane σ-complexes: insights on C–H bond activation

“…In fact, the carbon resonance in methonium is quite shielded compared to methane. Our result (À18.1 ppm) differs from the value reported by Rasul et al, 24 (À11.5 ppm) which was calculated at the MP2 level of theory. Nevertheless, with both levels of theory, a significant shielding, of about 10 ppm, of the methonium 13 C resonance is predicted compared to methane, despite the coordination to a positively charged entity.…”

Ab Initio/GIAO-CCSD(T) Calculated 13C–11B NMR Chemical Shift Relationships in Isoelectronic Hypercoordinate Carbonium and Boronium As Well As Carbenium and Borenium Ions

“…Prakash et al reported an extension of the relationship to cage compounds containing trivalent carbon and boron atoms . Olah and Prakash et al also derived a similar empirical equation for the hypercoordinate carbocations (carbonium ions) and their corresponding hypercoordinate boron compounds. , A similar empirical equation for the hypercoordinate hydrido carbocations and their isoelectronic boron compounds was developed by us …”

“…5,6 A similar empirical equation for the hypercoordinate hydrido carbocations and their isoelectronic boron compounds was developed by us. 7 We now report our study of the relationship between GIAO-CCSD(T) calculated 11 B NMR chemical shifts of a series of interesting boronium and borenium ions and the 13 C NMR chemical shifts of the corresponding isoelectronic carbonium and carbenium ions, respectively. Calculations of the accurate 13 C NMR chemical shifts of organic molecules by DFT methods have recently been reviewed.…”