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Ab Initio GIAO-MP2-Calculated Structures and B- C NMR Chemical

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Ab Initio GIAO-MP2-Calculated Structures and B- C NMR Chemical Ab initio͞GIAO-MP2-calculated structures and 11B-13C NMR chemical shift relationship in hypercoordinate onium-carbonium dications and isoelectronic onium-boronium cations Golam Rasul, G. K. Surya Prakash, and George A. Olah* Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park, Los Angeles, CA 90089-1661 Contributed by George A. Olah, May 15, 2002 The boronium–carbonium ion continuum was extended to include hypercoordinated onium-carbonium dications and the isoelec- tronic onium-boronium cation analogs. Structures and 13C and 11B NMR chemical shifts of the onium-carbonium dications and the corresponding isoelectronic and isostructural onium-boronium cat- ions were calculated with the ab initio͞GIAO-MP2 method. The data show a good linear correlation between 11B and 13C NMR chemical shifts, indicating that the same factors that determine the chemical shifts of the boron nuclei also govern the chemical shifts of carbon nuclei of these hypercoordinated onium ions and dications. oron and carbon are consecutive first-row elements. It Bfollows that electron-deficient trivalent carbocations are ␦11 ͑ ͒ ϭ ␦13 Ϫ isoelectronic with the corresponding neutral trivalent boron B BF3:OEt2 0.33 C͑TMS͒ 30 [2] compounds. Spielvogel et al. (1) and No¨th and Wrackmeyer (2) Recently, we have been able to show (6) a good boronium– have been able to show a close relationship between the exper- carbonium ion relationship for the hypercoordinated hydriodo imental 13C NMR chemical shifts of the carbons in carbocations 11 carbocations and their isoelectronic and isostructural boron and the corresponding experimental B NMR chemical shifts of ϩ compounds (e.g., parent five coordinated carbonium ion CH the boron atoms in isoelectronic boron compounds. The general 5 iii and five coordinated boronium ion BH iv) based on ab correlation equation for trigonal species is given in Eq. 1: 5 initio͞GIAO-MP2 calculations. These relationships show that the same factors that determine ␦11 ͑ ͒ ϭ ␦13 Ϫ B BF3:OEt2 0.40 C͑TMS͒ 46. [1] the chemical shifts of the boron nuclei also govern the chemical shifts of carbon nuclei. Based on the ab initio͞GIAO-MP2 In Eq. 1, the ␦13C is the chemical shift of the cationic carbon of calculations we now extend this boronium–carbonium contin- the carbenium ion with respect to tetramethylsilane, and the ␦11B uum to hypercoordinate onium-carbonium dications (v) and the is the chemical shift of the corresponding boron atoms with respect to the BF3:OEt2. The empirical Eq. 1 is in good agree- ment with most of the available data. Later, G.K.S.P. et al. (3) reported an extension of the relationship to cage compounds containing trivalent carbon and boron atoms. Williams et al. (4) derived a similar empirical Eq. 2 for hypercoordinate (5) carbocations (carbonium ions) and their CHEMISTRY corresponding hypercoordinate boron compounds (e.g., square ϩ pyramidal structure of C5H5 i and square pyramidal structure of pentaborane ii). *To whom reprint requests should be addressed. E-mail: [email protected]. www.pnas.org͞cgi͞doi͞10.1073͞pnas.142294599 PNAS ͉ July 23, 2002 ͉ vol. 99 ͉ no. 15 ͉ 9635–9638 Downloaded by guest on October 2, 2021 isoelectronic onium-boronium cations (vi), hitherto not yet Table 1. GIAO-MP2͞tzp͞dz-calculated and estimated NMR observed. chemical shifts a, ␦13B b, ␦11C b, ␦11B No. (GIAO-MP2) (GIAO-MP2) (estimated)* 1 Ϫ16.5 15.2 Ϫ25.0 2 Ϫ35.9 Ϫ18.6 Ϫ36.1 3 Ϫ2.7 50.9 Ϫ13.2 4 Ϫ23.3 Ϫ2.5 Ϫ30.8 5 Ϫ36.1 Ϫ8.1 Ϫ32.7 6 Ϫ19.4 22.2 Ϫ22.7 7 10.2 85.5 Ϫ1.8 8 Ϫ10.5 21.3 Ϫ23.0 9 Ϫ1.9 51.7 Ϫ12.9 10 Ϫ13.5 13.0 Ϫ25.7 Calculated 13C and 11B NMR chemical shifts were referenced to TMS and BF3:OEt2, respectively. *Estimated by using Eq. 2. H2O, H2S, CO, N2, FH, ClH, CO2, and CS2) 1-10b were also calculated at the same MP2͞6-311ϩG** level and found to be minima (Fig. 1). However, no attempts were made to locate the ϩ ϩ NH ,PH,HO, H S, HF, HCl, CO, N ,CO, and CS ) global minima for H4C X . Similar to boron, each of the ؍ X) 3 3 2 2 2 2 2 structure of 1-10b contains a hypercoordinate carbon involving a three-center two-electron bond. We have calculated the 13C and 11B NMR chemical shifts of 1-10 at the correlated GIAO-MP2 level (Table 1). The GIAO- Results and Discussion MP2 method has been shown to give reliable results for 11B ϭ The geometry optimizations were performed at the MP2͞6- NMR chemical shift calculations in H3BX (X NH3,PH3, ␦13 311ϩG** level with the GAUSSIAN 98 program package (7). H2O, H2S, and CO) complexes (15). The calculated C value Vibrational frequencies at the MP2͞6-31G**͞͞MP2͞6-31G** of dicationic 1b is 15.2 (with respect to tetramethylsilane) and level were used to characterize stationary points as minima the calculated ␦11B of cationic 1a is Ϫ16.5 (with respect to 13 ␦11 [number of imaginary frequency (NIMAG) ϭ 0]. The C and BF3:OEt2). We also estimated the B values in 1a (Table 1) 11B NMR chemical shifts were calculated with the MP2͞6- by applying Eq. 2 and by using the calculated ␦13C value in 1b 311ϩG** optimized geometries by the GIAO method (8–10). of 15.2. This yields a ␦11B value of Ϫ25.0 for the monocationic The GIAO-MP2 (11, 12) calculations with a tzp͞dz basis set boron compound 1a. The ␦11BofϪ25.0 deviates somewhat (11–13) have been performed with the ACES II program (14). The from the GIAO-MP2-calculated value of Ϫ16.5. The GIAO- 13 11 C NMR chemical shifts were referenced to (CH4)4Si [calcu- MP2-calculated ␦ B values of 2a, 5a, and 6a are Ϫ35.9, Ϫ36.1, lated absolute shift, i.e., ␴(C) ϭ 198.8]. The 11B NMR chemical and Ϫ19.4 respectively. The corresponding estimated ␦11B shifts were first computed with B2H6 [calculated absolute shift, values obtained by applying Eq. 2 are Ϫ36.1, Ϫ32.7, and Ϫ22.7, ␴ ϭ 11 i.e., (B) 96.7] as reference. The B NMR chemical shifts respectively, and they differ slightly from the GIAO-MP2 ␦ were finally referenced to BF3:OEt2 [ (B2H6) 16.6 vs. values. For other systems such a difference between GIAO- BF3:OEt2]. MP2-calculated and -estimated ␦11B values obtained by ap- We have previously calculated (15) the structures of proton- plying Eq. 2 is about 10 ppm. These results reconfirm the close ϩ ϭ ated borane-Lewis base complexes H4BX (X NH3,PH3, 11 13 ͞ relationship between the B and C chemical shifts of H2O, H2S, and CO) with the ab initio method at the MP2 6- ϩ isoelectronic boron and carbon analogs 1-10a and 1-10b, 31G** level. Protonations of H3BX to form H4BX were found ͞ respectively. to be highly exothermic (e.g., 193 kcal mol for H3BNH3). As expected there also seems to be a trend on the 11B and 13C Protonation seems to always take place on the B-H bond to form chemical shifts as a function of substituents. The 11B (or 13C) a three-center two-electron (3c-2e) bond. We have now recal- ϩ ϭ shifts of the ions are increasingly more deshielded with the culated the structures of H4BX (X NH3,PH3,H2O, H2S and ϭ increase of the electronegativity of the substituent atom. CO) 1-5a as well as additional structures (X N2, HF, HCl, CO2, ͞ ϩ and CS2) 6-10a at the MP2 6-311 G** level (Fig. 1). The 13 11 structures 1-10a were found to be the global minima at this level Correlation of C and B NMR Chemical Shifts. In general, we found ϩ a good linear correlation between the calculated 13C NMR of calculations. All of the monocations H4BX are B-H proton- ated involving hypercoordinate boron with a 3c-2e bond. The chemical shifts of studied onium-carbonium dications and the 11 structures 1-10a can also be considered as onium-boronium ions. calculated B NMR chemical shifts of the corresponding iso- The MP2͞6-311ϩG**-calculated structures of 9a and 10a agree electronic onium-boronium cation analogs (Fig. 2). The close well with the recently reported density functional theory relationship further demonstrates that even dicationic–cationic B3LYP͞6-311ϩG**-calculated structures (16). We have also systems closely follow Eq. 2 derived for nonclassical systems. The recently reported (17) the structures of 7a and 8a calculated at correlation line (3) derived from Fig. 1 closely corresponds to the the MP2͞6-311ϩG** level. correlation line (2). The corresponding isoelectronic and isostructural carbon ϩ ϩ ϭ ␦11 ͑ ͒ ϭ ␦13 Ϫ analogs onium-carbonium dications H4C X (X NH3,PH3, B BF3:OEt2 0.45 C͑TMS͒ 25 [3] 9636 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.142294599 Rasul et al. Downloaded by guest on October 2, 2021 CHEMISTRY Fig. 1. MP2͞6-311ϩG**-optimized structures of 1–10. Rasul et al. PNAS ͉ July 23, 2002 ͉ vol. 99 ͉ no. 15 ͉ 9637 Downloaded by guest on October 2, 2021 In conclusion, we have extended the11B-13C NMR chemical shift relationship of boronium-carbonium ions to include the hyper- coordinate onium-carbonium dications and isoelectronic onium- boronium cation analogs. With the correlated GIAO-MP2 method the 11B NMR chemical shifts of the boronium ions and the 13C NMR chemical shifts of their corresponding isoelec- tronic and isostructural carbonium dications were calculated. The data showed good linear correlation between the 11B and 13C NMR chemical shifts. This indicates that these onium- carbonium and onium-boronium ions also follow Eq.
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