ARTICLE pubs.acs.org/JPCA
Effect of Electronic Interactions on NMR 1JCF and 2JCF Couplings in cis- and trans-4-t-Butyl-2-fluorocyclohexanones and Their Alcohol Derivatives Pedro R. Anizelli,† Denize C. Favaro,† Ruben H. Contreras,‡ and Claudio F. Tormena*,† † ‡
Chemistry Institute, University of Campinas, Post Office Box 6154, 13084-971 Campinas, SP, Brazil Department of Physics, FCEyN, University of Buenos Aires and IFIBA-CONICET, Buenos Aires, Argentina
bS Supporting Information ABSTRACT: In order to study the influence of hyperconjugative, inductive, steric, and hydrogen-bond interactions on 1JCF and 2JCF NMR spinspin coupling constants (SSCCs), they were measured in cis- and trans-4-t-butyl-2-fluorocyclohexanones and their alcohol derivatives. The four isotropic terms of those SSCCs, Fermi contact (FC), spin dipolar (SD), paramagnetic spinorbit (PSO), and diamagnetic spinorbit (DSO), were calculated at the SOPPA(CCSD)/EPR-III level. Significant changes in FC and PSO terms along that series of compounds were rationalized in terms of their transmission mechanisms by employing a qualitative analysis of their expressions in terms of the polarization propagator formalism. The PSO term is found to be sensitive to proximate interactions like steric compression and hydrogen bonding; we describe how it could be used to gauge such interactions. The FC term of 2JCF SSCC in cis4-t-butyl-2-fluorocyclohexanone is rationalized as transmitted in part by the superposition of the F and O electronic clouds.
1. INTRODUCTION High-resolution nuclear magnetic resonance (NMR) parameters are excellent tools for determining electronic molecular structures, as has been described in many review articles15 and specialized books.610 The information about electronic structure from NMR spectra can be obtained through adequate analyses of chemical shifts and spinspin coupling constants (SSCCs), since in many cases they depend strongly on subtle aspects of the electron distribution between coupled nuclei.15,11,12 During the last decades an increase was observed in the systematic use of one- and two-bond SSCCs for studying conformations and configurations.1320 Although isotropic coupling constants are contributed from four terms—Fermi contact (FC), spin-dipolar (SD), paramagnetic spinorbit (PSO), and diamagnetic spinorbit (DSO) terms—many one- and twobond SSCCs are accepted to be mainly dominated by the FC term.2,5 However, for those involving at least one F atom, the SD and PSO contributions can also be very important.4,2126 In previous papers3,2729 the influence of hyperconjugative interactions on the FC terms of one- and two-bond SSCCs were studied. It was observed30 that geminal 2JXH (X = H, C, O, and N) couplings are sensitive to hyperconjugative interactions involving either bonding or antibonding orbitals belonging to their coupling pathways. A case in point for X = H and Y = C is 2JCH in cisand trans-4-tert-butylcyclohexanol (Figure 1), where its notable dependence on the OH conformation was rationalized as differences in hyperconjugative interactions involving the four bonds σC1C2, σC2C3, σC2Hax, and σC2Heq and their respective r 2011 American Chemical Society
antibonding orbitals, as symbolized in the scheme shown at the right-hand side of Figure 1. It is known that fluorinated compounds have a remarkable record in medicinal chemistry, and it is expected that they will continue playing an important role in providing new compounds for therapeutic applications. Rather small fluorinated molecules present a significant impact on drug development, because the fluorine atoms in a molecule can cause some effects such as, for instance, perturbation of pKa, modulation of lipophilicity, conformational changes, and electrostatic interactions, which are important aspects for a drug to be effective.31,32 It is known that the biological activity of a molecule depends notably on its electronic structure, which influences the physicochemical parameters for acting as a pharmaceutical agent, determining its interaction with a given protein.31,32 These considerations show how important it is to improve methods that provide insight into different aspects of electronic molecular structures from qualitative analyses of high-resolution NMR parameters. In this work the effects of carbonyl and hydroxyl groups in the transmission mechanisms for 1JCF and 2JCF SSCCs in cis- and trans-4-tert-butyl-2-fluorocyclohexanones (1 and 2) and their alcohol derivatives (36) (Figure 2) are taken as model compounds to show how the ideas mentioned above can be carried
Received: March 18, 2011 Revised: April 25, 2011 Published: May 12, 2011 5684
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Figure 1. cis- and trans-4-tert-Butylcyclohexanols and the 2JXY SSCC coupling pathway, which includes all four bonding and their corresponding antibonding orbitals.
Figure 2. Compounds 16 studied in this work.
out by resorting only to methods and concepts usually employed in most organic chemistry laboratories. This study includes both the experimental measurement of the quoted NMR parameters and their calculation within the secondorder polarization propagator approach (coupled cluster singles and doubles) [SOPPA(CCSD)]33 method where all four isotropic terms are considered. Whenever the trends of those terms in this series of compounds are considered relevant, they are analyzed in terms of their respective electronic molecular structures. Such analyses are carried out via a qualitative approach that has proved useful to get detailed insights into subtle aspects of electronic molecular structures.34 The method is described briefly in section 4, where adequate references are given to allow interested readers to follow it in detail. 1 JCF and 2JCF SSCCs are also calculated within the coupled perturbed density functional theory (CP/DFT)35 approach to verify whether such results follow similar trends to those obtained with the SOPPA(CCSD) method. This comparison aims at verifying whether such analyses could also be applied to larger molecular systems where the SOPPA(CCSD) method would be too demanding of computational resources.
2. EXPERIMENTAL SECTION 2.1. Computational Details. Geometries for compounds 16 were optimized at the B3LYP36/aug-cc-pVDZ37 level of theory by use of the Gaussian03 suite of programs.38 SOPPA(CCSD)SSCC39 calculations were performed with the Dalton 2.0 program40 employing the EPR-III basis set for carbons C1, C2, and C3 as well as for the F atom. For oxygen, hydrogen, and
the remaining C atoms, the cc-pVDZ basis set was employed. DFT SSCC calculations were carried out by use of the B3LYP functional in conjunction with the EPR-III41 basis set for all carbon, hydrogen, and fluorine atoms, while for the oxygen atom the cc-pVDZ37 basis set was used. These last calculations were carried out with the Gaussian03 suite of programs.38 In all compounds, natural bond orbital (NBO) parameters42 as well as expansions of canonical molecular orbitals (CMOs) in terms of NBOs were obtained with the NBO 5.0 program43 and their calculations were obtained at the B3LYP/ cc-pVTZ level by use of their optimized geometries quoted above. 2.2. Syntheses. 2.2.1. 4-tert-Butyl-2-fluorocyclohexanones 1 and 2.44. The reaction between 4-tert-butylcyclohexanone (1.8 g; 13.2 mmol) and 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2,2,6]octane bis(tetrafluoroborate) (4.6 g; 13.2 mmol) provided a mixture of isomers 1 and 2, which were separated by chromatography column (hexane/acetate 8:2). 2.2.2. 4-tert-Butyl-2-fluorocyclohexanols 3 and 4.44,45. The reaction between 4-tert-butyl-cis-2-fluorocyclohexanone (1) (370 mg; 2.2 mmol) and NaBH4 (0.09 g; 2.4 mmol) provided a mixture of isomers 3 and 4, which were separated by chromatography column (hexane/acetate 7:3). 2.2.3. 4-tert-Butyl-2-fluorocyclohexanols 5 and 6. The reaction between 4-tert-butyl-trans-2-fluorocyclohexanone (2) (370 mg; 2.2 mmol) and NaBH4 (0.09 g; 2.4 mmol) provided a mixture of isomers 5 and 6, which were separated by chromatography column (hexane/acetate 7:3). 2.3. NMR Experimental Details. The solvents were commercially available and used without further purification. 1H NMR spectra were recorded on spectrometers operating at 250 (62.5) and 300 (75) MHz for 1H (13C). Measurements were carried out 5685
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Table 1. Theoretical and Experimentala 1JC2F SpinSpin Coupling Constants for Compounds 16b 1
JC2F (Hz)
1 FC SD PSO
2
5
ax
6
eq
241.9
238.2
227.2
226.5
230.5
221.1
224.1
230.0
(287.9)
(274.3)
(274.1)
(271.1)
(267.8)
(273.2)
(279.1)
19.0
23.8
22.4
22.7
22.5
22.9
22.1
21.6
(20.5)
(25.9)
(24.6)
(24.8)
(24.9)
(25.0)
(30.4)
(23.4)
22.5
37.9
33.3
34.3
32.9
34.5
33.2
32.0
(19.8)
(35.6)
(31.4)
(32.0)
(32.0)
(31.9)
(24.1)
(29.4)
1.0
expc
4
(292.4)
DSO total
3
1.0
1.0
1.0
1.0
1.0
0.9
0.8
(1.0) 199.5
(1.0) 175.6
(1.0) 170.5
(1.0) 168.5
(1.0) 174.1
(1.0) 162.7
(0.9) 167.8
(0.9) 175.6
(251.1)
(225.4)
(217.3)
(216.3)
(213.2)
(209.9)
(217.8)
(225.4)
167.0d
172.2d
191.9
177.2
174.0
173.4
169.9
170.2
Experimental error is (0.5 Hz. Calculated values were obtained at the SOPPA(CCSD)/EPR-III and DFT/B3LYP/EPR-III levels. DFT-calculated values are shown in parentheses. c Only absolute values of these couplings were measured in this work. d Taken from ref 46. a
b
Table 2. Theoretical and Experimentala 2JC1F SpinSpin Coupling Constants for Compounds 16b
Table 3. Theoretical and Experimentala 2JC3F SpinSpin Coupling Constants for Compounds 16b
2
2
JC1F (Hz)
FC
JC3F (Hz)
1
2
3
4
5
6
ax
eq
14.8
22.6
15.7
16.1
26.8
16.7
20.1
17.9
FC
(15.0) (24.5) (16.8) (17.3) (31.4) (17.9) (20.5) (18.2) SD
0.9
1.0
(0.7) (1.0) PSO
3.1
6.6
0.7
0.7
(0.9) 1.6
0.5
(0.9) (0.5) 1.6
3.5
(4.4) (6.6) (1.5) (1.5) (4.1) DSO 0.1 0.3 0.2 0.2 0.2 total
0.6
0.3
(0.7)
(0.4)
0.2
1.4
(0.3)
(0.2)
(0.2)
(0.2)
(0.2)
(0.1)
(0.0)
15.3
15.0
15.4
23.0
18.0
19.4
15.3
13.5
21.0
16.9
17.2
17.7
SD
2.9
(0.1)
PSO
19.5
21.5c
3
4
5
6
ax
eq
17.4
19.0
16.7
16.2
18.1
19.0
20.1
17.9
0.6
1.0
0.4
0.3
0.8
0.7
0.6
0.3
(0.7)
(1.3)
(0.5)
(0.4)
(1.0)
(0.9)
(0.7)
(0.4)
2.8
(2.8) DSO 0.1 total
(27.0) (19.7) (20.5) (15.7) 29.2
2
(17.0) (19.0) (16.7) (16.6) (18.5) (19.8) (20.5) (18.2)
(0.5) (1.4) (3.0) 0.2 0.0 0.0
12.7
(11.4) (17.2) (16.4) exp
0.9 (1.1)
1
0.2
2.2
2.5
1.1
1.1
1.4
2.9
(1.0) (2.7) (2.8) (0.7) (0.8) (1.4) (3.0) 0.1 0.1 0.1 0.1 0.1 0.0 0.0
(0.1)
(0.1)
(0.1)
(0.1)
(0.1)
(0.1)
(0.1)
(0.0)
15.1
20.3
15.0
14.1
17.8
18.6
19.4
15.3
(15.0) (21.4) (14.6) (14.3) (18.9) (20.0) (20.5) (15.7)
17.7c
exp
17.3
22.1
17.5
16.8
20.8
20.7
21.5c
17.7c
Experimental error is (0.5 Hz. Calculated values were obtained at the SOPPA(CCSD)/EPR-III and DFT/B3LYP/EPR-III levels. DFT-calculated values are shown in parentheses. c Taken from ref 46
Experimental error is (0.5 Hz. Calculated values were obtained at the SOPPA(CCSD) /EPR-III and DFT-B3LYP/EPR-III levels. DFTcalculated values are shown in parentheses. c Taken from ref 46.
with a probe temperature of ca. 25 C, using solutions of ca. 10 mg/cm3 in CDCl3 and CD2Cl2. Typical conditions for 1H spectra were 16 transients, spectral width 2.2 kHz, with 32K data points, giving an acquisition time of 7.4 s and zero-filled to 64K to give a spectral resolution of 0.1 Hz/point. For 13C spectra the conditions were 500 transients, spectral width 18 kHz, with 64K data points, giving an acquisition time of 1.84 s and zero-filled to 128K to give a spectral resolution of 0.5 Hz/point. 19F{1H} NMR spectra were recorded at 25 C on a Bruker DPX 250 spectrometer, operating at 235.3 MHz for 19F, using solutions of ca. 15 mg/cm3 in CDCl3. Typical conditions for 19F spectra were 16 transients, spectral width 15 kHz, with 128K data points, giving an acquisition time of 4.5 s and zero-filled to 128K to give a spectral resolution of 0.1 Hz/point. The spectra were referenced to external CFCl3.
experimental values are compared with those calculated within the SOPPA(CCSD)/EPR-III as well as those calculated within the B3LYP/EPR-III approaches. In Table 1 are collected values for 1JC2F SSCCs for compounds 16 as well as for fluorocyclohexane in the axial and equatorial conformations. In Tables 2 and 3 are collected values corresponding to 2JC1F and 2JC3F, respectively. The following features of data reported in Table 1 are worth mentioning. In these 1JC2F SSCCs the FC, SD and PSO terms are significant. In all cases the FC term is negative, in agreement with other data published in the current literature. This renders total 1 JC2F SSCCs negative, being SOPPA(CCSD) results in agreement with experimental values within (5 Hz with the exceptions of compounds 1 and 6, where that limit increases to (8 Hz. Total DFT calculated absolute values are overestimated by about 50 Hz in comparison with experimental SSCCs. Trends of total calculated values, within both the SOPPA(CCSD) and DFT approaches, are approximately the same and follow rather closely the experimental one, except for compound 5, where, as observed in Table 1, such discrepancy originates in the FC term. Probably,
a
b
3. RESULTS In order to detect significant changes in either 1JCF or 2JCF SSCCs along the series of compounds 16, the respective
a
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Table 4. Experimental and Theoretical Substituent Chemical Shifts, 19F Chemical Shifts, and Theoretical Nuclear Magnetic Shielding Constants for Compounds 16a 1
a
2
3
4
5
6
ax
eq
SCS (exp), ppm
23
þ1.5
14.7
15.7
þ0.3
17.6
SCS (theor), ppm δ (19F), ppm
17.2 188.0
þ8.6 185.5
11.6 179.7
13.4 180.7
þ2.0 185.7
17.0 203.6
186b
165b
σ, ppm
370.7
363.8
365.1
366.9
370.4
389.4
372.4
353.5
Values for fluorocyclohexane in axial and equatorial conformations are also displayed. b Taken from ref 48.
Table 5. Some Interatomic Distances from Optimized Geometries of Compounds 16
F---O, Å
1
2
2.6876
3.3092
3
4
5
6
2.7712
2.8442
3.5686
2.7773
F---HOH, Å
2.3473
2.4577
4.2910
2.3545
OH, Å
0.9634
0.9631
0.9645
0.9639
this discrepancy originates in a medium effect on the experimental value since it is noted that, in the alcohol compounds, 5 is the only one where the OH group is not quite close to the F atom. Apparently, this would facilitate specific interactions between solute molecules. Also, calculated trends for each individual coupling term are the same within both approaches. It is noteworthy that noncontact terms in the four alcohols are almost insensitive to the OH and F relative orientations. The DSO terms are the same for all 1JC2F SSCCs couplings considered in Table 1. This result is in agreement with well-known features of the total isotropic DSO value.47 Experimental trends of total calculated 2JC1F SSCCs (Table 2) and of 2JC3F SSCCs (Table 3) are, in general, slightly better reproduced at the DFT-B3LYP/EPR-III than at the SOPPA(CCSD)/EPR-III levels. Since the FC term of 2JC1F SSCC along the 16 series presents interesting features, they are considered below in some detail. On the other hand, the absolute values of the SD contributions to 2JC1F SSCCs are too small to attempt any detailed rationalization. Differences observed in 2JC3F SSCCs along the 16 series are, in general, not very significant although a few of them are worthy of note. Some comments are made below. In Table 4 are compared experimental and calculated 19F substituent chemical shifts (CSCs) for compounds 16, where it is observed that, in general, there is very good agreement between them. For this reason, it is considered that the level of theory employed to calculate these SCSs is adequate for the purpose of this work. It is noted that for alcohols 3, 4, and 6 these SCSs correspond to a shielding effect. As discussed below, in these three compounds there is a close proximity between the HOH and the F atom and this shielding effect seems to originate in this F---H interaction.
4. DISCUSSION Some structural parameters obtained from the optimized geometries are depicted in Table 5. In alcohols 3, 4, and 6, the F---H distance is shorter than the sum of the respective van der Waals radii, (1.47 þ 1.20) Å = 2.67 Å. This result indicates that there is an interaction between the OH and F moieties that yields a slight lengthening on the OH bond length when compared with that in compound 5. The NBO analysis does not show any interaction of type LP(F) f σ*OH with strength
above the NBO program threshold, that is, 0.5 kcal/mol. This suggests that such hydrogen-bond interactions are dominated by electrostatic interactions. On the other hand, the O---F distance in compounds 1, 3, 4, and 6 is also shorter than the respective sum of the van der Waals radii, (1.47 þ 1.52) Å, indicating that in all four compounds there is a significant overlap between the O and F electronic clouds. How these proximate interactions—that is, the F---O lonepair overlap in 1, 3, 4, and 6 and the F---H hydrogen-bond interactions in 3, 4, and 6—affect the respective 1JCF and 2JCF SSCCs is studied in this work. These as well as other aspects of the electronic molecular structures of 16 are studied by resorting to qualitative analyses presented in previous papers.34 Only a brief description of this qualitative model is given here; adequate references are given for interested readers. Within these qualitative analyses, second-order terms of SSCCs (FC, PSO, and SD terms) are expressed as sums of contributions involving two occupied and two virtual orbitals. To this end, expressions given within the polarization propagator49 approach are employed. 4.1. Fermi Contact Term. The FC contribution to nJCF SSCCs (n = 1, 2) can be written as an expansion of terms depending on pairs of occupied (i, j) and virtual (a, b) molecular orbitals (MOs): n FC J CF
¼ ΩFC
∑ nJ FCia, jb ðCFÞ
ia, jb
ð1Þ
where ΩFC is a constant involving the coupling nuclei magnetogyric ratios as well as universal and numerical constants. As shown previously,49d MO contributions to the FC term can be written as in eq 2: n FC J ia, jb
¼ 3 W ia, jb ½UiaFC, C UjbFC, F þ UiaFC, F UjbFC, C
ð2Þ
1 where 3Wia,jb = (3A þ 3B)ia,jb are the elements of the inverse of the triplet polarization propagator matrix (PP) and matrices 3A and 3B can be written in terms of bielectronic molecular integrals. For the present purpose it is enough to recall that diagonal elements of the PP matrix, 3Wia,ia, within this qualitative approach can be described as being almost inversely proportional to the energy gap involving virtual a and occupied i MO energies. FC UFC ia,jb (Ujb,F) are the FC “perturbators”, that is, the matrix elements of the FC operator between the occupied i (j) and virtual a (b) MOs evaluated at the C (F) site of the coupling nuclei (eq 3), where δ(r BN) is the Dirac δ function:
UiaFC, N ¼ Æijδð B r N Þjaæ where N ¼ C, F
ð3Þ
This is the overlap between occupied (i) and unoccupied (a) localized molecular orbitals (LMOs) at the site of the N nucleus, which is given by the product of i and a LMOs s % character at the N atom. 5687
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Table 6. NBO Parameters Needed To Calculate the Descriptors for Bond and LPm Contributions for 1JC2F Spin Spin Coupling Constants in 16 Series. a 1 occ(σCF)
a
1.9944
2
3
4
5
Table 7. Descriptors for 1JB and 1JLP1(F) a for the FC Term of 1 JC2F SpinSpin Coupling Constants in Compounds 16
6
1.9911
1.9933
1.9933
1.9929
1.9934
s % C(σCF) 19.88
18.43
17.79
17.85
18.02
17.65
s % F(σCF) 26.79
25.34
26.56
26.45
26.40
26.18
occ(σ*CF)
0.0256
0.0349
0.0382
0.0377
0.0323
0.0416
Δε LP1(F)
1.166 32 1.108 03 1.109 83 1.109 30 1.116 08 1.096 71 1.9922 1.9926 1.9921 1.9922 1.9921 1.9923
s%
72.77
73.78
72.84
73.03
72.74
72.90
LP2(F)
1.9683
1.9730
1.9739
1.9741
1.9729
1.9742
s%
0.30
0.44
0.56
0.46
0.57
0.89
LP3(F)
1.9651
1.9676
1.9703
1.9704
1.9713
1.9706
s%
0.12
0.41
0.02
0.04
0.27
0.01
Orbital energies are given in hartrees.
Equations 2 and 3 provide for the FC term of a given nJCF SSCC a useful qualitative description of its trend along a series of compounds, which could be affected by either the “perturbators” or the 3Wia,jb matrix elements or both. By use of adequate approximations, it is possible to estimate qualitatively how inductive, conjugative, hyperconjugative, and steric interactions affect those SSCCs. Such approximations provide adequate “descriptors” to rationalize the nJCF SSCC trends along a given series of compounds. These approximations are based on the following considerations. (1) Equations 2 and 3 are invariant under unitary transformations; therefore, MOs involved in their expressions can be considered to be localized MOs (LMOs). (2) It is assumed that occupied LMOs behave under interactions quoted above like core, bonding, or lone-pair orbitals, while virtual LMOs behave like antibonding or Rydberg orbitals of the NBO approach of Weinhold and co-workers42 3 W matrix elements are largest for diagonal elements, and the second largest 3W matrix elements are “quasi”-diagonal; that is, either the two occupied or the two vacant orbitals are equal to each other, and the other pair is equal to the corresponding diagonal element. Diagonal 3W matrix elements 3Wia,ia in eq 3 decrease, in absolute value, whenever there is an interaction that increases the energy gap between the corresponding a antibonding and i bonding NBOs, Δε = εa εi. Vice versa, interactions that reduce such gaps yield an increase in its absolute value. This proportionality implies having an unknown factor whose changes, along a given series of compounds, are expected to be smaller than those introduced by either the energy gap or the perturbators. When we consider the behavior analogy between LMOs and NBOs in eq 3, it is recalled that in the latter each s % character is the same for both bonding and antibonding orbitals. To take into account this property, s % characters are weighed with the respective orbital occupancies, obtaining “effective” s % characters. For n = 1, that is, 1JCF SSCCs, and 3Wia,ia diagonal elements, in eq 2 both terms within the square brackets are equal to each other. The corresponding factor 2 is no longer shown since it is included in the unknown factor quoted above. In a previous paper,49d the main two types of contributions in eq 2 for 1JCF SSCCs were studied and it was concluded that quasi-diagonal terms of the 3W matrix associated with lone pairs overcome that of the diagonal one since the corresponding perturbators reverse
a
1
2
3
4
5
6
FC (Hz)
241.9
238.2
227.2
226.5
230.5
221.1
D(JB) D(JLP1)
6.3 20.1
9.5 30.7
11.7 35.5
11.3 34.7
8.4 25.8
13.4 40.9
As given in eqs 4 and 5. SOPPA(CCSD) FC terms are also shown.
the PP trend. These two types of contributions are (1) the “bond contribution”, 1JB, where i = j = σCF and a = b = σ*CF, and (2) the “lone-pair contributions” (corresponding to quasi-diagonal terms, and therefore their dependence on the energy gap can be neglected), where i = σCF 6¼ j = LPm(F) and a = b = σ*CF, with m = 1, 2, 3, where LPm(F) are numbered as in the NBO method, that is, from deepest to highest orbital energies. Since the LP1(F) s % character is much larger than those of LP2,3(F), in this analysis 1JLP2 and 1JLP3 terms are neglected in comparison with 1 LP1 J . In Table 6 are collected NBO parameters needed for applying this qualitative approach to rationalize the FC trend of 1JC2F SSCCs along the 16 series, shown in Table 1. They are used to define “descriptors” for each contribution, 1JB and 1JLP1(F). These are given explicitly in terms of NBO parameters in eqs 4 and 5, respectively: J ¼ ½occðσ CF Þ2 ½s%Cðσ CF Þ2 ½occðσ CF Þ2 ½s%Fðσ CF Þ2 =Δε
1 B
ð4Þ
1 LP1ðFÞ
J
¼ ½occðσ CF Þ½occðσ CF Þ2 ½s%Cðσ CF Þ2 ½occðLP1 Þ½s%FðLP1 Þ½s%Fðσ CF Þ
ð5Þ 1 B
1 LPm(F)
NBO parameters necessary for estimating the J and J “descriptors” for compounds 16 are collected in Table 6. Descriptors for 1JB and 1JLP1(F) for the FC term of 1JC2F SSCC in compounds 16 as calculated from eqs 4 and 5 are given in Table 7. It is recalled that 1JB involves a diagonal element of the 3 W matrix, while 1JLP1(F) involves a quasi-diagonal element of the same matrix, the former being notably larger than the latter. For this reason 1JB descriptors cannot be compared directly with 1 LP1 J descriptors. It is noted that the FC trend along the 16 series is almost described by the 1JB descriptors, although the influence of the 1JLP1 descriptor is also noted, as for instance in the FC term of 1JC2F in 4. The power of this approach can be appreciated by having a quick look at eqs 4 and 5 and Table 6, where it is observed that the more positive FC term in 6 originates mainly in the large σ*CF occupancy. Although 1JLP2,3(F) are neglected in comparison with 1JLP1(F), the s % character of LP2(F) and LP3(F) experience proportionally large changes along the 16 series (Table 6). As noted recently,34h for compounds 1, 3, 4, and 6, such changes reflect the F---O and F---H proximity interactions and they should affect the PSO term of 1JC2F SSCCs. Therefore, a brief qualitative analysis of this term follows. It should be noted that the proportionally large decrease of the LP3(F) s % character in compounds 3, 4, and 6 is in line with the negative increase (shielding effect) of the 19 F SCS in Table 4. 4.2. Paramagnetic SpinOrbit Term. The only calculated term of this contribution is the isotropic one, although the PSO term is a second-rank tensor. When rationalizing how intramolecular 5688
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Table 8. Four Bonding and Four Antibonding Orbital Occupanciesa as Descriptors of Hyperconjugative Interaction Effects on the FC Term of 2JC1F SpinSpin Coupling Constants Displayed in Table 3
Figure 3. Coordinate axes used to describe the PSOR(F) vector corresponding to the PSO isotropic term of 1JC2F SSCC in compounds 16. a
interactions affect this term, its tensor character must be considered. The PSORR diagonal component contributing to the 1 JCF tensor holds an expression similar to eq 149d for the FC term: PSO, R PSO, R 1 PSO;RR J CF ¼ ΩPSO Uia, C 1 W ia, jb Ujb, F ð6Þ ia, jb
∑
where ΩPSO is a constant involving the magnetogyric ratios of the coupling nuclei as well as universal and numerical constants. 1 Wia,jb = (3A þ 3B)1 ia,jb are the elements of the inverse of the singlet PP matrix. As in the 3W matrix, the largest elements of the 1 W matrix are “diagonal”, that is, they satisfy i = j and a = b. However, the diagonal terms of 1W are accompanied by very small perturbators, and therefore the corresponding energy gaps are not relevant. The focus will be centered on the corresponding PSOR perturbator, which shows vector character. Their expressions are given in eqs 7a and 7b; they correspond to eq 2 for the FC term. ! PSO, R r F r ÞR rF 3 jaæ ð7aÞ Uia, F ¼ Æijð B PSO, R
Ujb, C
! ¼ Æjjð B r C r ÞR rC 3 jbæ
ð7bÞ
It is noted that when LMOs are employed, diagonal 1W matrix elements play very minor roles since even for 1JFC SSCCs they are associated with very small perturbators. Similarly, 1W quasidiagonal elements play an important role for 1JFC SSCCs. In other cases the associated perturbators are too small to be considered in a qualitative approach of this type. For rationalizing how proximity interactions in 1, 3, 4, and 6 affect the isotropic PSO term of 1JC2F SSCCs, the vector given in eq 7a is considered since it must be the perturbator most affected by such interactions. Approximations similar to those considered above when analyzing the FC term are introduced. i, j (a, b) are supposed to be occupied (vacant) LMOs and their behavior under different interactions is assumed to be similar to those of the NBOs of Weinhold and co-workers.42 The vector PSOR (eq 7a) is referred to the coordinate axes shown in Figure 3, that is, along the three lone pairs belonging to the fluorine atom. In eq 7a (r BF rB)R is the 90 rotation operator around the R axis and centered at the F atom. Each PSOR component has a substantial value if an F lone pair rotated 90 around the R axis overlaps significantly with an antibonding orbital. Thus, the PSOX component is neglected, while the Z component is positive and the Y component is negative. Descriptors analogous to those for the FC term, eqs 4 and 5, are written for the PSOY,Z
compd
bonding
antibonding
FC (Hz)
C1 s %
C1---F (Å)
1
7.912
0.181
14.8
32.26
2.4050
2
7.929
0.171
22.6
31.60
2.3812
3
7.935
0.146
15.7
27.14
2.3840
4
7.933
0.143
16.1
27.31
2.3830
5
7.929
0.171
26.8
26.70
2.3812
6
7.936
0.142
16.7
27.07
2.4025
Shown at the right-hand side of Figure 1.
components: For PSOY :
½s%ðLP3 Þ½occðLP3 Þ½s%FðσCF Þ½occðσCF Þ½ 1=rF 3 æ
ð8aÞ
½s%ðLP2 Þ½occðLP2 Þ½s%FðσCF Þ½occðσ CF Þ½ 1=rF 3 æ
For PSOZ :
ð8bÞ where Æ1/rF æ is a kind of mean electronF nucleus distance in integral 7a. It is observed that this factor is dominant in eqs 8a and 8b since changes in the LP2,3 s % character are proportionally the largest. When that s % character increases, the ÆrFæ values decreases and [Æ1/rF3æ] increases, and vice versa. Therefore, it is expected that trends in eqs 8a and 8b are by far dominated by the [Æ1/rF3æ] factor. Therefore, these LP3 and LP2 s % characters are the main parameters to take into account in this qualitative analysis of the PSOR perturbator (R = X, Z). Since in 1 the CdO bond is not parallel to the CF bond, the steric F---O effect is expected to affect both LP3(F) and LP2(F) s % character. Values displayed in Table 6 support that assumption. In fact, the respective s % characters when going from compound 2 to compound 1 change LP2(F) from 0.44 to 0.30 and LP3(F) from 0.41 to 0.12; that is, the proportion of the reduction in the negative Y component is notably smaller than that of the positive Z component. In short, the isotropic part of PSO term must be smaller in 1 than in 2. The SOPPA(CCSD) calculation shown in Table 1 is in line with this qualitative approaches since this is reduced from 37.9 Hz in 2 to 22.5 Hz in 1, although such difference could originate also in other types of interactions. Interestingly; the PSO term of 1JC2F provides a sensitive parameter to gauge F---O interactions affecting the F atom. Such steric interactions seem to yield a decreases in the s % character of LP2(F) and LP3(F). Tentatively, it can be assumed that attractive interactions, like hydrogen bonding (HB), would increase the s % character of the lone pair involved in that interaction. However, there could be a further effect on the vector in eq 7a that could be ascribed to the antibonding orbital corresponding to the bond undergoing HB. In fact, this could happen according to eq 7a, when a 90 rotation of LP1(F) around the Y axis is considered. This would correspond to a positive contribution. (see Figure 3). This could happen in compounds 3 and 4, where the LP2(F) s % character is notably smaller than in 6. It must be noted that changes in the LP1 s % are, proportionally, much smaller than those observed in LP2(F) and LP3(F) for compounds 16. In the three alcohols, 3, 4, and 6, where not only F---O interactions take place but also an F---H hydrogen bond operates, the Z contribution increases, due in part to the increase of LP2 s % 3
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The Journal of Physical Chemistry A Table 9. Occupied Canonical Molecular Orbitals That Define Coupling Pathways for the FC Term of 2JC1F in Compounds 1 and 2a compound 1, FC = 14.8 Hz
compound 2, FC = 22.6 Hz
MO 23 (occ): ε = 0.556 522 au
MO 23 (occ): ε = 0.555 131 au
0.345*: BD (1) C1C2
0.282*: BD (1) C1C2
0.303*: LP1(F)
0.243*: LP1F
0.271*: BD (1) C2F
0.233*: BD (1) C2F
MO 27 (occ): ε = 0.478 853 au
MO 26 (occ): ε = 0.489 308 au
0.318*: BD (1) C1C2 0.302*: BD (1) C1C6
0.344*: BD (1) C2F 0.278*: LP1F
0.268*: LP(3)F(lp)
0.251*: BD (1) C1C6
0.239*: LP2O 0.230*: BD (1) C2F MO 75 (vir): ε = 0.282 887 au
MO 75 (vir): ε = 0.273 907 au
0.322*[237]: BD*(1) C1C2*
0.367*[237]: BD*(1) C1C6*
0.266*[238]: BD*(1) C2F*
0.235*[236]: BD*(1) C2F*
a
Only the relevant NBOs are shown. Their whole expansions are shown in the Supporting Information.
character, which increases the positive Z component, and in part to the decrease of LP3(F) s % character, which decreases the absolute value of the negative Y component. When LP3 s % character is compared along the alcohol series, the lowest value is observed for compound 5, where no F---H hydrogen bonding takes place. This suggests that the F---H hydrogen bond (attractive interaction) increases slightly the LP3 s % character. It is also observed that, when steric F---O compression is increased upon going from 2 to 1, also the s % character of LP3 decreases. These trends are associated with the effect discussed many years ago by Lee and Chesnut.50 4.3. 2JC1F and 2JC3F SpinSpin Coupling Constants. The four components of 2JC3F SSCCs along the series 16 do not show significant changes to attempt their rationalization in detail (Table 3). Therefore, in this section only some rationalization on the FC term of 2JC1F SSCCs is carried out. In a previous paper30 the influence of hyperconjugative interactions on the FC term of geminal SSCCs was described, considering the coupling pathway shown at the right-hand side of Figure 1. The occupancies of the four bonds and those of the four antibonding orbitals shown in Figure 1 are summed up in compounds 16; these values are displayed in Table 8. However, this simplified scheme must be complemented with other considerations since they alone cannot provide a consistent rationalization for describing the FC term of 2JC1F in Table 2, where proximity interactions are envisaged when the geometrical parameters shown in Table 5 are considered. For instance, if an electronegative atom (or group) is bonded to C1, then according to Bent’s rule51 in the C1C2 bond the s % character at C1 changes without a concomitant change in hyperconjugative interactions. According to eq 3 this increase of the s % character at C1 would yield an increase in 2JC1F SSCC (“inductive effect”); for this reason in Table 8, column 5, are also displayed the C1 s % values. It is also noted that in all compounds 16 the C1---F distance, taken from the respective optimized structures (Table 8, column 6) is in all cases notably shorter than the respective sum of their van der Waals radii, (1.70 þ 1.47) Å. This suggests that for all these compounds several FC coupling pathways for 2JC1F SSCC could be operating. It is noted that in
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alcohols 3 and 4 the FC term of 2JC1F SSCCs as well as other parameters displayed in Table 8 are practically the same. Looking for other possible FC coupling pathways in 2JC1F SSCCs for compounds 16, this analysis is complemented with a recently published method dubbed FCCP-CMOs, which stands for Fermi contact coupling pathways studied with canonical molecular orbitals.34f Briefly, this approach is based on eqs 1 and 2, where, instead of the approximations presented above, the canonical molecular orbitals (CMOs) are analyzed. It is highlighted that CMOs satisfy Pauli’s exclusion principle, and therefore the Fermi hole can span its spatial region. Consequently, a similar assertion holds for the spin information associated with a given coupling constant. With this approach, diagonal elements of the 3W matrix become the most relevant terms. To get an idea about the spatial extension of CMOs, the program NBO 5.043 is employed for expanding them in terms of NBO orbitals. Details of the FCCP-CMOs method are given in ref 34f. It is highlighted that to carry out studies with this method requires only the use, without any modification, of two common computational quantum chemistry programs. In the Supporting Information section are shown the NBO expansions of the main CMOs defining the FC transmission pathways for 2JC1F SSCCs in compounds 16. To illustrate an interesting example, in Table 9 are displayed occupied CMOs participating in the FC transmission corresponding to 2JC1F SSCCs in compounds 1 and 2, since they are notably different, that is, their FC terms are, respectively, 14.8 and 22.6 Hz. In Table 9 are shown for each of them two occupied and one vacant CMO, which represent two FC coupling pathways. The two main virtual transitions in 1 are CMO-23 f CMO-75* and CMO-27 f CMO-75*, while in 2 they are CMO23 f CMO-75* and CMO-26 f CMO-75*. The former are similar in both compounds and no further comment is made. The latter show differences like, for instance, in compound 1 the following pathways are envisaged. The simultaneous presence of both the σC1C6 bond and the LP3(F) suggests that the proximity effect C1---F is not efficient for transmitting the FC term of 2JC1F due to the very low s % character of LP3(F). On the other hand, the simultaneous presence of LP2(O) and LP3(F) suggests that their overlap defines a pathway F---O---C1. This pathway is similar to that described recently for 2JC2Hf SSCC in the furfural syn rotamer.52 Apparently, in 1 such contributions should be negative to describe correctly the 2JC1F SSCCs experimental trend.
’ CONCLUDING REMARKS The qualitative analyses applied in this work to study the title compounds have provided interesting insight into the transmission mechanisms for the FC and PSO terms of one- and twobond JCF couplings. Of particular interest is the analysis of the effect of proximate interactions on the PSO term of 1JCF SSCCs. Results obtained in this work suggest that attractive interactions on LP1(F) and LP2(F) increase their s % character, while repulsive interactions decrease that s % character. It would be interesting to extend to more general cases the validity of such tentative conclusions. Work along this line is currently under way, and in a forthcoming paper such results will be reported. It is highlighted that it could be misleading to study only the increase or decrease of the PSO term since, although the usually calculated amount is scalar, this qualitative analysis requires one to take into account its second-rank character. 5690
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The Journal of Physical Chemistry A In compound 1 it is envisaged that for 2JC2F SSCC an unusual coupling pathway for its FC term takes place. A similar coupling pathway was observed previously for the syn conformation in 5-substituted furfurals.52 This coupling pathway is studied by the FCCP-CMO approach (Fermi contact coupling pathways studied by analyzing canonical molecular orbitals). It is highlighted that this approach requires only two well-known quantum chemistry programs without any modification, namely, Gaussian 0338 or beyond, and NBO 5.0.43
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional information on Occupied Canonical Molecular orbitals that define coupling pathways for the FC term of 2JC1F in compounds 1 to 6. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail
[email protected].
’ ACKNOWLEDGMENT We are grateful to FAPESP (Grants 08/06282-0 and 10/ 10993-9) for financial support of this work and to CNPq for scholarships (to P.R.A. and D.C.F.) and a fellowship (to C.F.T.). R.H.C. gratefully acknowledges economic support from CONICET (PIP 0369) and UBACYT (X047). ’ REFERENCES (1) Krivdin, L. B.; Contreras, R. H. Annu. Rep. NMR Spectrosc. 2007, 61, 133. (2) Contreras, R. H.; Peralta, J. E.; Giribet, C. G.; Ruiz de Azua, M. C.; Facelli, J. C. Annu. Rep. NMR Spectrosc. 2000, 41, 55. (3) Contreras, R. H.; Peralta, J. E. Prog. NMR Spectrosc. 2000, 37, 321. (4) Contreras, R. H.; Barone, V.; Facelli, J. C.; Peralta, J. E. Annu. Rep. NMR Spectrosc. 2003, 51, 167. (5) Juaristi, E.; Cuevas, G. Acc. Chem. Res. 2007, 40, 961. (6) Grant, D. M.; Harris, R. K. Encyclopedia of Nuclear Magnetic Resonance; Wiley: Chichester, U.K., 1996. (7) Pople, J. A.; Schneider, W. G.; Bernstein, H. J. High-Resolution Nuclear Magnetic Resonance; McGraw-Hill: New York, 1959. (8) Emsley, J. W.; Feeney, J.; Sutcliffe, L. H. High-Resolution Nuclear Magnetic Resonance Spectroscopy; Pergamon: Oxford, U.K., 1966. (9) Kutzelnigg, W.; Fleischer, U.; Schindler, M. NMR—Basic Principles and Progress, Vol. 23; Springer: Heidelberg, Germany, 1990. (10) Webb, G. A. Annual Report on NMR Spectroscopy; Academic: London, 1985. (11) Gakh, Y. G.; Gakh, A. A.; Gronenborn, A. M. Magn. Reson. Chem. 2000, 38, 551. (12) Kaupp, M.; Bulh, M.; Malkin, V. G. In Calculation of NMR and EPR Parameters, Theory and Application; WileyVCH: Darmstadt, Germany, 2004. (13) Church, T. J.; Carmichael, I.; Serianni, A. S. J. Am. Chem. Soc. 1997, 119, 8946. (14) Cloran, F.; Zhu, Y.; Osborn, J.; Carmichael, I.; Serianni, A. S. J. Am. Chem. Soc. 2000, 122, 6435. (15) Cloran, F.; Carmichael, I.; Serianni, A. S. J. Am. Chem. Soc. 2001, 123, 4781. (16) Stenutz, R.; Carmichael, I.; Windmalm, G.; Serianni, A. S. J. Org. Chem. 2002, 67, 949.
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