Transmission of polar substituent effects across the cubane ring system: ¹⁹F substituent chemical shifts (SCS) of 4‐substituted (X) cub‐1‐yl fluorides revisited

Abstract

Abstract19F NMR shieldings of 4‐substituted (X) cub‐1‐yl fluorides (4) for a set of substituents (XH, NO2, CN, NC, CF3, COOH, F, Cl, HO, NH2, CH3, Si(CH3)3, Li, O− and NH) covering a wide range of electronic substituent effects were calculated using the DFT‐GIAO theoretical model. The level of theory, B3LYP/6‐311+G(2d,p), provided 19F substituent chemical shifts (SCS) in good agreement with experimental values where known. By means of NBO analysis, various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated 19F SCS and polar field, resonance and group electronegativity substituent constants (σF, σR and σx, respectively) and also the NBO derived molecular parameters (fluorine natural charges (Qn), electron occupancies on fluorine of lone pairs (nF) and occupation number of the CF antibonding orbital (σCF*)). The key determining parameters appear to be nF and σCF*(occup). Both factors are a function of the electrostatic field influence of the substituent (σF effect) but are counteractive in their influence on the shifts. No evidence for a significant resonance effect influence on the shifts could be identified. Copyright © 2009 John Wiley & Sons, Ltd.

Computational 19F NMR. 1. General features

“…Some early computational work (aimed at benchmarking and validation) included F 2 as test molecule, which proved to be challenging [26][27][28]. Interest in modeling 19 F shifts has steadily continued, and several papers have appeared in the last decade aimed at general structural issues, solution chemistry and physical organic chemistry, [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] solid-state issues (especially the prediction of chemical shift tensors), [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] J-couplings [65][66][67][68][69][70][71][72]…”

A DFT‐GIAO and DFT‐NBO study of polar substituent effects on NMR 17O chemical shifts in some rigid polycyclic alkanes

“…The calculated 17 O SCS (ppm) of 1 – 6 (Y = OH) are listed in Table 1. In order to facilitate comparisons, the previously published corresponding data for the fluorides 1 – 6 (Y = F)5, 11, 14 are also listed (Table 2). A cursory scrutiny of the data (Tables 1 and 2) reveals that overall the SCS of the various alcohols and fluorides have similar signs with the numerical values of the former being smaller than the latter.…”

“…Furthermore, the 17 O and 19 F SCS of the various systems ( 1 – 6 ) correlate linearly for each system with reasonable precision except for system 4 ( 1 , r = 0.98; 2 , r = 0.94; 3 , r = 0.95; 4 , r = 0.48; 5 , r = 0.95; 6 , r = 0.77; 6 (excluding O − and NH), r = 0.98). Thus, based on the previous detailed regression analyses of the 19 F SCS of 1 – 6 (Y = F),5, 11, 14 an obvious corollary is that a σ χ effect(s) is the dominant factor underlying the 17 O SCS of 1 – 3 and 5 – 6 This is confirmed by correlations of the 17 O SCS of 1 – 6 (Y = OH) against polar substituent parameters ( σ F and σ χ ; polar field and electronegativity effect, respectively) by means of regression analysis. The values of σ F and σ χ deployed are given in Table 3 whilst a summary of the regression parameters is set out in Table 4.…”

“…Hence, the signs of the 19 F SCS of 3 (Y = F) parallel those of remotely substituted aryl fluorides which are known to be governed by the perturbation of the π‐electrons on fluorine 13. A similar investigation of the 19 F SCS of the 4‐substituted(X)‐cub‐1‐yl fluoride system ( 4 , Y = F)14 has also implicated the importance of the π‐electrons on fluorine. However, in the latter system the electron population of the CF antibonding orbital ( σ * CF (occup)) is also significant.…”

Applications of nuclear shielding

“…453 19 F NMR shieldings of 4-substituted (X) cub-1-yl fluorides (4) for a set of substituents (X = H, NO 2 , CN, NC, CF 3 , COOH, F, Cl, HO, NH 2 , CH 3 , Si(CH 3 ) 3 , Li, O-and NH) covering a wide range of electronic substituent effects were calculated using the DFT-GIAO theoretical model. 454 Orientational order properties of two fluorinated liquid crystals containing the 1,3,2-dioxaborinane ring are studied by optical, dielectical and 13 C and 19 F NMR spectroscopies. 455 Solid-state guest dynamics of tetratolyl tetraurea calix [4]arene tetrapentylether dimeric capsules filled with different types of aromatic guests such as benzene-d 6 , fluorobenzene-d 5 and 1,4-difluorobenzene are studied.…”