1JCH Coupling in Benzaldehyde Derivatives - ACS Publications

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Article Cite This: ACS Omega 2019, 4, 1494−1503

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JCH Coupling in Benzaldehyde Derivatives: Ortho Substitution Effect

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Angelita Nepel, Renan V. Viesser, and Cláudio F. Tormena* Institute of Chemistry, University of CampinasUNICAMP, P.O. Box 6154, 13083-970 Campinas, SP, Brazil

ACS Omega 2019.4:1494-1503. Downloaded from pubs.acs.org by 37.44.252.130 on 01/20/19. For personal use only.

S Supporting Information *

ABSTRACT: The natural J-coupling (NJC) method is applied to analyze the Fermi contact contribution of the NMR spin−spin coupling constant decomposing this contribution in terms of natural localized molecular orbitals. We investigated the influence of the basis set on the NJC analysis for the formyl group coupling constant (1JCHf) of benzaldehyde derivatives. NJC and other NBO analyses, like steric and natural Coulombic energy, were chosen to explain the influence of electron-donating and electron-withdrawing groups on 1JCHf for some substituted benzaldehydes (Me, OH, OMe, F, Cl, Br, I, and NO2). For the ortho derivatives, electronegative substituents near the C−Hf bond increase the 1JCHf coupling. This effect could be related to an increase in formyl carbon s character and changes in the carbon and hydrogen natural charges. This indicates that the substituents in ortho have a proximity effect on 1JCHf coupling mainly of electrostatic origin instead of the expected hyperconjugative interactions.

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INTRODUCTION Nuclear Magnetic Resonance (NMR) is the most powerful spectroscopic technique for obtaining detailed chemical information in solution being widely applied for structure elucidation. The fundamental parameters such as chemical shift (δ) and scalar coupling constant (J) are the most important sources of information to achieve unequivocal molecular structure assignment and can also be applied to determine the conformation adopted by a molecule in solution.1,2 As δ and J are sensitive to the electronic environment experienced by the nuclei, both can be used as a probe to evaluate small changes in the electronic structure.3 Quantum mechanical calculations are usually required to support the NMR experimental measurements providing interesting information about how the stereoelectronic interactions contribute to nuclear magnetic shielding and spin−spin coupling constant (SSCC) transmission mechanisms.3−8 Among the different types of SSCCs (one-bond, two-bond, three-bond, and long-range SSCC), the one-bond SSCC stands out for providing important information about the nature of the chemical bond.9 The one-bond SSCC is an indirect interaction between two atoms bonded; therefore, the bond length and the s character of the involved atoms are often invoked to explain conformational and substituent effects on this type of coupling constant.9−12 For example, it is expected, for molecular systems with the same connectivity, that 1JCH decreases with bond lengthening but increases when s character for the spn carbon hybrid orbital rises.9,13 These relationships allow us correlate the 1 JCH values with hybridization and stereoelectronic effects such as hyperconjugative and conjugative interactions.3,9,12,14 The Perlin effect13 is probably the most known effect that correlates the 1JCH with bond length. The smaller 1JCH shown for axial hydrogens (Haxial) in cyclohexane and some derivatives are © 2019 American Chemical Society

usually assigned as a result of hyperconjugative interactions between occupied (lone pair, π or σ orbitals) and unoccupied σ*C−Haxial orbitals in an antiperiplanar relationship.13−16 These interactions tend to lengthen and weaken the C−Haxial bond decreasing the 1JCH. However, the hyperconjugative origin of the Perlin effect has been discussed mainly in the work of Cuevas et al.,17 which attributed that the increase in the 1JCH value in HCOC fragments arises from dipolar interactions. Moreover, electrostatic effects have also been suggested to explain 1JCH variations for hydrogen-bonded complexes, where it was observed that negative charges near the H atom could increase 1 JCH.18−20 These observations indicate that 1JCH is not in a direct simple way related to bond lengths; therefore, other factors such as electrostatic and rehybridization effects should be considered.9 Information about the origin of SSCC transmission mechanisms could be obtained through decomposition analyses. A variety of methods have been proposed to decompose the SSCCs into orbital contributions to clarify which are the main interactions that influence the SSCCs transmission.21−28 Among them, the natural J-coupling (NJC) is often applied to understand the role of steric and hyperconjugation effects on the transmission mechanisms of the SSCCs.3,29−32 The NJC, an NBO-based method, is employed to analyze the Fermi contact (FC) portion of the SSCC. The NJC splits the FC contribution in terms of individual natural localized molecular orbitals (NLMOs), which can provide insights into the electronic structure of molecules once each NLMO contribution Received: October 31, 2018 Accepted: January 8, 2019 Published: January 17, 2019 1494

DOI: 10.1021/acsomega.8b03035 ACS Omega 2019, 4, 1494−1503

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Figure 1. Benzaldehyde derivatives under investigation, where X = H, F, Cl, Br, I, OH, OCH3, NO2, and CH3.

the influence of the basis set choice on NJC analysis for 1JCHf is investigated by testing different combinations of polarization and diffuse functions. We also apply the natural steric and natural Coulombic energy (NCE) analysis35 to investigate changes on the L term of the SSCCs according to steric or electrostatic components.

is decomposed into Lewis (L) and non-Lewis (NL) terms. These terms allow assessing the “steric” or “hyperconjugative” origin of the SSCC once the former is usually associated with the L term of each NLMO and the latter is related to orbital delocalization. A complete description and some applications of the NJC method can be found in the original article published by Wilkens et al.30 This approach was successfully applied to illustrate the angular and distance dependences of the vicinal 3 JHH coupling constants in ethane and long-range 6JHH in pentane. It was demonstrated that these coupling constants are dominated by the Lewis term.30 A study with similar interest was proposed to investigate the torsional dependence and substituent effect for ethane and fluoroethane.32 Also, NJC has been used to understand complex effects, such as the Perlin effect, which was deeply explored on oxocane derivatives16 by our research group. These examples demonstrate NJC as an important approach to understand the transmission mechanisms of the SSCC in several organic compounds. An important precaution when working with NBO analyses is the proper basis set choice. A detailed investigation about the use of very large basis sets with augmented diffuse functions in NBO analyses has already been published and it presents anomalies on energies results.33 Therefore, it is not surprising if any NBO-based results could be liable to the influence of the basis set choice. In a previous work about basis set dependence on NJC analysis,30 the authors did not observe any important influence of the basis set choice, so they suggested that the choice of the atomic basis set has no appreciable effect on the decomposition results. On the other hand, another study reported anomalous values on FC decomposition of 3JHH coupling when BS2 basis set, developed specially for J-coupling calculations, was used on NJC analysis.32 For larger coupling constants, like the results published for 1JCF using the EPR-III basis set,3 it was observed that Rydberg (Ry) contributions were important to describe the transmission of 1JCF coupling. However, Rydberg orbitals are included in the NBO scheme to ensure orbital orthogonality and, according to NBO theory, it is not expected a significant role of this kind of orbitals in the analysis.30,33,34 So, a physical interpretation of Rydberg contribution prevents reliable conclusions about the transmission mechanism of the SSCCs and that result could be only an outcome of an unsuitable basis set choice for this kind of analysis. Based on the difficulty to rationalize the effect of stereoelectronic interactions on 1JCH and how the basis set chosen can affect the NJC analysis, the present study aims to evaluate the effect of substituents on the 1JCH of substituted benzaldehydes using the density functional theory (DFT). In this paper, ortho-, meta-, and para-substituted benzaldehydes were chosen as a model system to verify how electron-donating and electronwithdrawing substituents could affect 1JCHf (Figure 1). These molecules allow us to evaluate the role of σ- and π-systems and also how the substituent position influences 1JCHf, mainly the proximity effect in ortho-substituted compounds. Furthermore,

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RESULTS AND DISCUSSION Overall Trends for 1JCHf. Figure 2 shows the experimental results of 1JCHf for all benzaldehyde derivatives studied. These

Figure 2. Experimental 1JCHf coupling constants difference (Δ1JCHf) relative to benzaldehyde (174.0 Hz) for all compounds under investigation.

data are displayed as a difference (Δ1JCHf) relative to benzaldehyde to allow an easier comparison, and the corresponding values are presented in Table S2 in the Supporting Information (SI). A significant increase of about 10 Hz (18 Hz for nitrosubstituted) on the SSCC is observed when the substituents are in ortho position, except for hydroxyl and methyl groups, where a small increase (2.4 Hz) and a slight decrease (1.0 Hz) are detected, respectively. This effect was intensified with the presence of a fluoride group in position 2 for the compounds substituted in positions 2 and 6. On the other hand, meta- and para-substituted benzaldehydes show similar values (except for nitro) of 1JCHf as observed for the benzaldehyde. These experimental evidences suggest that the spatial proximity between C−Hf bond and substituents in ortho position is more important for the 1JCHf transmission pathway than the electron-donating or electron-withdrawing character of the substituents. Theoretical SSCC calculated with DFT indicated that 1JCHf is totally described by the FC term (Table S3), what is expected for a one-bond SSCC. This was corroborated by applying the SOPPA(CCSD)/aug-cc-pVTZ-J theoretical level for benzalde1495


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Figure 3. Fermi contact (FC) contributions (Hz) to the 1JCHf of benzaldehyde. The total FC and its contributions are relative to the experimental 1JCHf applying the following correction factors: D95 (1.0202), 6-31+G* (1.0872), cc-pVDZ (1.0437), 6-311G** (1.1217), 6-311++G** (1.1312), aug-ccpVDZ (1.0543), cc-pVTZ (1.1413), aug-cc-pVTZ (1.1320), aug-cc-pVTZ-J (0.9268), EPR-III (1.0342), and pcJ-2 (0.9452).

Figure 4. (a) Main differences of the FC term and its orbital contributions (Hz) between o-chorobenzaldehyde and benzaldehyde. (b) Main differences of the σC−Hf NLMO contributions to the FC term between o-chorobenzaldehyde and benzaldehyde.

hyde 1JCHf SSCC calculation (1JCHftotal = 170.4 Hz; 1JCHfSD = 0.2 Hz; 1JCHfPSO = −1.3; 1JCHfDSO = 1.0 Hz; 1JCHfFC = 170.5 Hz). To understand the origin of the FC term increase on 1JCHf, the NJC methodology was applied. For this purpose, first, a basis set dependence investigation was carried out. Basis Set Dependence on NJC Analysis. Although three functionals, namely, PBE, PBE0, and BHandH, have been used, only results applying the PBE0 functional are presented in the main text for the benzaldehyde and o-chorobenzaldehyde, which were chosen to highlight our findings related to basis set dependence on the NJC decomposition analysis. The results

obtained with PBE and BHandH functionals follow a similar trend despite the difference in absolute values (Tables S4−S12). To check the influence of the basis set on NJC analysis, 11 different combinations of polarization and diffuse functions were tested to decompose the FC term into L and NL contributions for benzaldehyde. Once the absolute values of the FC term and its contributions could change significantly according to different combinations of functional and basis set, a correction factor was applied to the results of each basis set to adjust the FC term according to the experimental 1JCHf. Using the corrected FC term, it is possible to evaluate how the ratio of L to NL 1496


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The 6-31+G* was the worst Pople basis set describing the total FC difference between the o-chlorobenzaldehyde and benzaldehyde, probably due to lack of polarization function to consider p orbitals at hydrogen atoms causing inaccuracies in the SSCC calculation. For the remaining basis sets, it is clear that Ry contributions are more pronounced when larger basis set augmented with diffuse functions are used, preventing a correct description of the contribution responsible for the transmission of 1JCHf SSCC for these two similar compounds. Among the basis sets used for SSCC calculations, the EPR-III, a basis set developed for FC description on EPR calculations, resulted in the worst FC decomposition because the total FC term was mainly described by Ry contributions. The Sauer’s (aug-ccpVTZ-J) and Jensen’s (pcJ-2) basis sets, specially developed for SSCC calculations, and the last one optimized for DFT functional methods showed significant Ry contributions; however, the FC term is mainly explained by the L term. Considering that the simplest double-ζ Dunning (D95) basis set showed a better description of L and NL contributions and a good correlation with experimental 1JCHf SSCC, this basis set would be suitable for performing NJC analysis to pick up the steric and hyperconjugative effects, without incurring the potential artifacts associated with diffuse functions. We also evaluated the performance of a long-range correlated functional (CAM-B3LYP) and a functional with OPTX exchange function (OPBE) with the basis set D95 and 6-311+ +G** (Tables S14−S16). Tables S14 and S15 show that longrange correlation could be important since CAM-B3LYP slightly improves the 1JCHf obtained with B3LYP. The OPBE performance was worse than PBE, indicating that OPTX exchange function is not suitable for coupling constant calculation in comparison to PBE exchange function. As the other functionals employed in this work, despite the differences obtained between DFTs, the effect of the functional is not as intense as the effect of the basis set for the CAM-B3LYP and OPBE as well. Table S16 shows that B3LYP, CAM-B3LYP, PBE, and OPBE provide the same conclusions about the increase of Ry contributions on the 1 JCHf difference between o-chlorobenzaldehyde and benzaldehyde using basis augmented with diffuse functions (6-31+ +G**). After the rationalization about a basis set appropriated to perform NJC analysis, the PBE0/D95 level of theory was applied to evaluate the substituent effect in ortho, meta, and para positions on 1JCHf for the benzaldehyde derivatives (Figure 1). For a better presentation of the results, mono- and disubstituted benzaldehydes are discussed separately. Monosubstituted Benzaldehydes. The FC values, calculated by the NJC approach, are in good agreement with the experimental data, indicating an increase of 1JCHf for most ortho-substituted benzaldehydes (Table S17) and no significant change for meta- and para-substituted benzaldehydes, except for nitro substituent, which is discussed later on. For halogens and methoxy groups in ortho position, anticonformers are more populated (>87%) than syn-conformers; therefore, the observed SSCCs are described mainly by anticonformers (Figure 1). For o-hydroxy, o-halo, and omethoxybenzaldehyde, the anti-conformers show 1JCHf bigger than benzaldehyde (Table S17). However, for the o-hydroxy substituent, the theoretical and experimental SSCCs do not have a significant increase in comparison to benzaldehyde. For this compound, the conformer syn is the most stable (Figure 1), due to the high stability of the intramolecular hydrogen bond of the hydroxyl group with the formyl oxygen. In the case of the ortho

(repolarization, valence antibonding, and Rydberg) contributions changes according to the atomic basis set used. The correction factors were calculated by dividing the experimental 1JCHf of the benzaldehyde by the total FC obtained using each basis set. This correction was possible since the theoretical SSCC is dominated by the FC term. While the original data are presented in the SI, Figure 3 shows the data multiplied by the correction factor for each basis set. According to Figure 3, it is possible to observe that L contributions (in purple) are always positive, while the NL ones are negative, reminding that NL is the sum of the repolarization (red), valence antibonding (blue), and Rydberg orbital (green) contributions. Only the D95 and 6-31+G* do not show significant Rydberg (Ry) contributions. An anomalous profile of these contributions was observed when the atomic basis set 6311++G** is used, i.e., a large negative Ry and a much higher positive L contribution was observed. Interestingly, L and NL contributions change significantly according to the atomic basis set used, but in all cases, the same trend is observed, i.e., positive and negative values for the L and NL contributions, respectively. The main deviations are observed on the ratio for the valence antibonding and Ry orbitals contributions. Therefore, for studies of SSCCs transmission mechanism applying the NJC methodology, it is important to choose a proper basis set, consistent with the NBO methodology, to get results with contributions mainly distributed into the valence orbitals that is the main idea of NBO analysis. The NJC analysis is usually performed to comparative purposes.30 Thus, to be considered chemically useful, the NJC analysis must be sensitive and consistent to changes between two or more molecules, not just evaluating the absolute contributions values for one structure. Therefore, the total FC contribution for the benzaldehyde was compared to ochlorobenzaldehyde (Figure 4a). In this case, no correction factor was applied since we are also evaluating the accuracy of the basis set describing the FC difference between ochlorobenzaldehyde and benzaldehyde. Even though the absolute values differ considerably according to the theoretical level applied, the difference of FC term for 1 JCHf coupling between o-chlorobenzaldehyde and benzaldehyde (Tables S10−S12) is very similar for all functionals tested with the same basis set. Except for 6-31+G*, the FC difference (ΔFC term) for all basis set described satisfactorily the experimental 1 JCHf difference between o-chlorobenzaldehyde and benzaldehyde. For all basis sets tested, it was also observed that the increase in the total FC contribution is described by σC−Hf NLMO (Figure 4b and Table S13), which is mainly formed by the coupled nuclei. In general, a better agreement between experimental and theoretical difference in 1JCHf (∼8.8 Hz) was obtained when the D95, cc-pVDZ, and 6-311G** basis sets were applied (Figure 4a). For these basis sets, the Ry contribution to total FC term is less than ±1.0 Hz. These three basis sets are not augmented with diffuse functions, indicating that NJC analysis is strongly affected by the addition of these types of functions, which agrees with Goodman and Sauers’ work.33 According to these authors, NBO methods based on orbital partitioning, as is the case of NJC, are more susceptible to basis set issues, i.e., diffuse functions can lead to artifacts of linear dependence and numerical instability. 1497


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methyl substituent, neither syn- nor anti-conformer showed 1JCHf higher than benzaldehyde (Table S17). These observations suggest that the proximity of electronegative substituents to C−Hf bond increases 1JCHf and indicate that the change in the electron density of the aromatic ring, caused by electron-withdrawing (e.g., NO2) or electrondonating (e.g., OMe) groups in meta and para positions, does not affect the SSCC studied. Independently of the electronic character of the substituents in ortho position, an increase in 1 JCHf was observed for substituents containing lone pair orbitals available. Therefore, lone pairs may affect 1JCHf by means of space interaction with C−Hf bond instead of the π-system delocalization. Through-space interactions promoted by ortho substituents are already known in the literature. For example, Zeidan et al.36 and Alabugin et al.37 while studying Bergman reactions observed that the cycloaromatization kinetics is affected by electronic, steric, and electrostatic effects caused by ortho substituents. The NJC analysis shows that the increase in 1JCHf for orthosubstituted benzaldehydes occurs mainly on the L term of the σC−Hf NLMO contribution, while the NL term shows very close values in comparison to benzaldehyde (Table S18). These results strongly corroborate that hyperconjugative contribution (NL term) is not involved in the transmission of 1JCHf for studied systems. It is not simple to describe the L contribution to the SSCCs, but usually for vicinal SSCCs, this term is associated with steric repulsion.30 However, in addition to a steric component, electrostatic effects could also contribute to the Lewis energy.35 Some observations about the molecular electronic structure are in accordance with the Lewis term increase, such as the increase of s character on the carbon atom of C−Hf bond (about 1%) (Table S19). The increase of s character on the carbon of C−Hf bond leads to a direct increase in the FC component, which is described by s electrons near coupled nuclei, and consequently in total 1JCHf SSCC (Figure 5a). Many factors could be associated with the increase of carbon s character. For example, an increase of Hf−C−C bond angle and a slight shortening of the C−Hf bond length (Table S19) were observed for all ortho-substituted benzaldehydes (except for omethyl). This could be a response to the steric interaction between the C−Hf orbital and the substituent’s lone pairs (Table S19). Rehybridization can also be observed by the carbon s character of CO and C−C bonds. The interactions between the C−Hf orbital and the substituent’s lone pairs or methyl C−H bonds could result in an electronic rearrangement to accommodate the electronic density into internal orbitals due to the proximity of substituent. However, despite the C−H orbitals of the methyl substituent show a steric repulsion with the C−Hf bond orbital, this compound does not show a similar effect to the remaining orthosubstituted compounds. This behavior suggests that this probably is not the main reason for the increase in the 1JCHf observed (Figure 2 and Table S19). The 1JCHf increase could also be explained by a C−Hf···X hydrogen bond. In this case, once the NL contributions do not have appreciable influence on FC increases, the weak LPX → * interactions have no significant contributions for the FC σC−Hf term. On the other hand, the electrostatic interaction resulting from C−Hf bond polarization could be related to carbon s character increase.38 The carbon s character may increase in improper hydrogenbonded complexes as a consequence of Bent’s rule, which

Figure 5. Differences between monosubstituted benzaldehydes and benzaldehyde. (a) σC−Hf NLMO Lewis (L) contributions for 1JCHf and formyl carbon (Cf) s character of σC−Hf NBO; (b) σC−Hf NLMO L contributions for 1JCHf and L VNCE between the formyl carbon and L hydrogen atoms (VC−Hf NCE ); and (c) formyl carbon (Cf) s character of C−Hf L σC−Hf NBO and VNCE .

predicts that if the hydrogen atom becomes less electronegative, in response to changes on atomic charges, the s character carbon 1498


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atom may increase. The C−Hf bond shortening (Table S19), generally known as blue-shifted hydrogen bond, is a consequence of the formyl carbon rehybridization.39 In the present work, new approaches to visualize how the charge or electronegative substituents affect the bond between coupled nuclei were used. Applying the qualitative natural Coulombic energy (NCE) analysis,35 it was possible to evaluate L and NL contributions to the natural charges separately and apply it to estimate an L or NL natural Coulombic energy (VL/NL NCE ). Particularly, in this study, this methodology was carried out to estimate the Lewis VLNCE of L the formyl carbon and hydrogen pair (VC−Hf NCE ). This strategy L was chosen because the main idea was to evaluate the VC−Hf NCE 1 L influence on the L term of JCHf. VNCE was estimated using the following equation qCqH L VNCE = |R C − RH| (1)

JCHf is observed. The effect of intramolecular hydrogen bonding on the 1H chemical shift and J between hydroxyl and aldehydic hydrogens was previously observed by Schaefer et al.40 For the o-nitrobenzaldehyde, 1JCHf coupling is 18.8 Hz larger in comparison to benzaldehyde, while for meta- and paranitrobenzaldehydes, 1JCHf increases around 5 Hz (Table S17). The NJC analysis for these compounds indicated that an additional increase originated from the L term of the adjacent σC−C bond between the formyl and ipso carbons (Table 1). Table 1. Main NLMO Contributions for the FC Term Calculated at the PBE0/D95 Level of Theory for syn- and anti-Conformers of the ortho-, meta-, and paraNitrobenzaldehyde in Comparison to Benzaldehyde

where qC and qH are the Lewis charges for the formyl carbon and hydrogen (in atomic unit), respectively, and |RC − RH| is the bond length (in Bohr). VLNCE could be converted to kcal mol−1 by applying the conversation factor of 627.51. In Figure 5, the differences of σC−Hf NLMO contributions for the 1JCHf of benzaldehyde derivatives compared to benzaldehyde L are related to the carbon s character (Figure 5a) and VC−Hf NCE (Figure 5b), and a comparison between the s character and C−Hf L VNCE difference is shown in Figure 5c. Analyzing the L difference in the VC−Hf of the formyl carbon and hydrogen NCE pair among the benzaldehyde derivatives in relation to benzaldehyde, it was possible to visualize that in the cases where the ortho substituents possess lone pairs near the C−Hf L bond, there is a great increase in VC−Hf (more than 200 kcal NCE −1 L mol ). In contrast, VNCE is not affected in methyl substituent (Figure 5b and Table S19). Once the L contribution of the FC term refers to the spin density, it is expected, especially for one-bond SSCC, that a large charge difference between the coupled nuclei represents a large spin polarization of the electronic system. Therefore, for the systems evaluated in this work, the results are consistent with the suggestion that Coulomb electrostatic interactions are the dominant effect caused by ortho substituents, which is responsible for the increase of 1JCHf on benzaldehyde derivatives instead of steric interactions. As discussed above, it was verified that Coulomb electrostatic effects could be related to the increase in 1JCHf and once both L VC−Hf and s character on the carbon of C−Hf bond increase NCE when the L term of the C−Hf bond contribution increases. The decrease in the natural L charge of the formyl carbon (less positive) (Table S19) could be related to the increase in the carbon s character (Figure 5c). An exception was observed in o-hydroxybenzaldehyde, where the syn-conformer is the only stable conformer and no reasonable change in 1JCHf or the carbon s character is observed, C−Hf L but a higher ΔVNCE was estimated. This principle of estimation could be explained by the occurrence of the intramolecular hydrogen bond between the formyl oxygen and the hydroxyl group (CO···H−O). The formyl carbon is directly bonded to the oxygen that participates as acceptor on hydrogen bond. The L natural charge of the carbon becomes more positive, instead of the cases where an increase in 1JCHf is observed, which contributes to a larger natural charge difference between the formyl carbon and hydrogen atoms, resulting in L high estimated value of ΔVC−Hf NCE , but any reasonable effect on

compound

Δ1JCHf theor.

Δ1JCHfFC

ΔLtotal

ΔσC−HfL

ΔσC−CL

ortho-(anti) ortho-(syn) meta-(anti) meta-(syn) para

20.9 10.0 5.6 4.8 5.4

20.8 9.9 5.7 4.8 5.5

20.7 9.3 5.9 4.9 5.5

15.2 0.0 −1.3 1.3 −1.8

5.3 7.8 7.4 3.8 7.5

The role of electrostatic effects on 1JCHf is supported by the correlation between the Swain−Lupton inductive-field parameter, dubbed F parameter,41,42 and the coupling constant values of ortho-substituted benzaldehydes (Figure S1). F is a known field-effect parameter estimated for several substituents. Halogens and methoxy and methyl groups showed a linear correlation with the F parameter (Figure S1B), indicating that substituent effect on the coupling constant values may be associated with field effect transmitted through space. Only hydroxyl and nitro substituents displayed deviations from linearity (Figure S1A). The stability of syn-conformer by intramolecular hydrogen bonding, as mentioned above, is the reason of deviation observed for hydroxyl substituent, while contributions of σC−C bond increase the 1JCHf and probably are not taken into account for F value of nitro group. Disubstituted Benzaldehydes. As in monosubstituted benzaldehydes, for the 2,6-disubstituted benzaldehydes, an increase of around 13 Hz on experimental 1JCHf (Figure 2) was observed in comparison to benzaldehyde. For 2,6-difluoro- and 2,6-dichlorobenzaldehydes, only one conformer is present, while for 2-chloro-6-fluoro-, 2-bromo-6-fluoro-, and 2-fluoro-6iodobenzaldehydes, both syn- and anti-conformers have a similar population in equilibrium and contribute similarly to the total theoretical 1JCHf (Table S17). In 2,6-disubstituted benzaldehydes, there is an electronegative halogen substituent near the formyl C−O and C−Hf bonds; therefore, steric repulsion between CO···X and C−Hf···X competes. Consequently, the Hf−C−C bond angle does not increase as in the ortho-monosubstituted benzaldehydes (Table S18). Hence, in 2,6-disubstituted benzaldehydes, the C−Hf bond is always spatially closer to the adjacent substituent than in ortho-monosubstituted compounds, resulting in a more pronounced proximity effect on the SSCC. In the same way as for ortho-monosubstituted benzaldehydes, the increase in 1JCHf occurs on the FC term (Table S3) and is mainly explained by the L contribution of the σC‑Hf NLMO (Table S18). In addition, these larger contributions could be also related to the increase in the s character (Figure 6a) and to L VC−Hf (Figure 6b) as explained before in monosubstituted NCE topic. 1499


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Figure 6. Differences between disubstituted benzaldehydes and benzaldehyde: (a) σC−Hf NLMO Lewis (L) contributions for 1JCHf and formyl carbon L (Cf) s character; (b) σC−Hf NLMO L contributions for 1JCHf and L VNCE between formyl carbon and hydrogen atoms (VC−Hf NCE ); and (c) formyl carbon L . (Cf) s character of σC−Hf NBO and VC−Hf NCE

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EXPERIMENTAL AND COMPUTATIONAL DETAILS

NMR Measurements. The 1JCHf SSCCs of substituted benzaldehydes were measured directly in the 1H NMR spectra using the satellite peaks from the protons bonded to 13C. These direct measurements were possible once the formyl signals were not overlapping with any other signal of the compound. The 1H NMR spectra were recorded in a Bruker Avance III spectrometer of 14.1 T equipped with a TBI probe. All spectra were acquired with a FIDRES less than 0.2 Hz, leading to an error around ±0.1 Hz on coupling constant measurements. Samples were prepared as solutions of 10 mg of solute in 0.6 mL of CDCl3. Computational Details. All calculations were carried out using the Gaussian 09 package,43 unless stated otherwise. Geometry optimizations, electronic potential, and Gibbs freeenergy calculations of benzaldehyde derivatives were carried out by applying M06-2X functional44 and 6-311G* basis set for iodine atom and aug-cc-pVTZ for the remaining atoms. These calculations were performed applying the SMD solvation model45 for chloroform (ε = 4.7113). M06-2X was the DFT selected for optimization and frequency calculations because it is a functional designed to predict the thermochemical properties of compounds containing main-group elements, being recommended to obtain accurate structures and energies.44,46 All calculations were carried out using default parameters implemented in Gaussian 09, i.e., tight convergence of 10−8 in the SCF procedure and fine integration grid. All SSCCs were calculated using the default approach for SSCC that gives the four terms of the Ramsey nonrelativistic theory: FC, spin−dipole (SD), paramagnetic spin−orbit (PSO), and diamagnetic spin−orbit (DSO). Theoretical 1JCHf SSCC for benzaldehyde was calculated applying the ab initio method SOPPA(CCSD)47−49 and the Sauer’s basis set aug-cc-pVTZ-J, which have been designed for magnetic resonance calculations. However, for all SSCC calculations, a DFT method, which requires less computational resource, was employed. The PBE0 method, 50 which is being used successfully on SSCC calculations,8,51−53 in combination with aug-cc-pVTZ-J basis set (6-311G* for iodine and aug-cc-pVTZ for bromine) was chosen. This theoretical level demonstrated good agreement with experimental results, and also, for benzaldehyde, the results

This article has investigated the effects of substituents on the 1 JCHf transmission mechanism of benzaldehyde derivatives. Most of the substituents showed a significant increase of 10 Hz on the 1JCHf in ortho position, while small or no effects were observed for meta and para derivatives. Our theoretical results indicated that the spatial proximity between C−Hf bond and substituents in ortho position is more important for the 1JCHf transmission mechanism than the electron-donating or electronwithdrawing character of the substituents. Applying the NCE analysis, it was possible to access the Lewis natural charges of the coupled atoms and verify that the increase in 1JCHf for ortho-substituted benzaldehydes is related to a decrease of natural charges on carbon and an increase in hydrogen. The C−Hf bond polarization and formyl carbon rehybridization lead to a C−Hf shortening and indicate that the proximity interaction is an improper hydrogen bond between the C−Hf bond and the ortho substituent. For nitro compounds, an additional increase was observed also for the meta and para compounds, which originates from the Lewis term of the adjacent C−C bond between the formyl and ipso carbons. This work also discussed, for the first time, the basis set effect on NJC analysis of the FC term for the 1JCHf coupling of benzaldehydes. It was observed that the ratio of the valence antibonding and Rydberg orbital contributions change according to the basis set employed. Basis augmented with diffuse functions (6-31+G*, 6-31++G**, aug-cc-pVTZ, aug-cc-pVTZJ, and EPR-III) showed significant Ry contributions, which indicates that this basis set could not be adequate for NJC analysis, to get results with contributions mainly distributed into the valence orbitals. This study also demonstrates that basis sets specially developed for SSCC calculations are not necessarily compatible with the NJC methodology being, simple basis set without diffuse or contracted functions, less vulnerable to linear dependence issues. The simplest dunning basis set D95 showed good results on NJC decomposition as well as describing satisfactorily the FC term of the investigated compounds. 1500


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tional resources; and Institute of Chemistry of UNICAMP for NMR facilities.

were equivalent to results obtained with the ab initio theoretical level described above. For comparative purpose with the NJC analysis, the SSCCs were also calculated applying the D95 basis set (6-311G* for iodine and aug-cc-pVTZ for bromine). We also calculated the 1JCHf changing the angle between formyl and phenyl groups (from 0 to 30° for anti-conformer and 150 to 180° for syn-conformer) for benzaldehyde, ochlorobenzaldehyde, and o-nitrobenzaldehyde because the formyl group is not coplanar to the phenyl ring for some ortho-substituted compounds. Table S1 shows that the torsion does not significantly change the 1JCH, but the syn- and anticonformers, in ortho-substituted benzaldehydes, contribute with different 1JCH values to the total SSCC. Therefore, all properties showed in this work (SSCC, NJC contributions, and NCE charges) were weighted by the individual conformer populations (anti and syn) estimated using the Gibbs free energies. For the NBO analysis:54 NJC, NBO, and NCE calculations, the functional PBE0 was used in combination with the D95 basis set, except for iodine and bromine atoms that are not described by this basis set and, therefore, the 6-311G* and aug-cc-pVTZ basis sets were selected, respectively. The NJC calculations were performed using a Fermi contact spin perturbation of 0.02 au. In addition, the basis set study for NJC analysis was conducted using three distinct functionals (PBE, PBE0, and BHandH) in combination with the following basis set: D95,55 6-31+G*,56,57 cc-pVDZ, 58,59 6-311G**, 60,61 6-311++G**, 60,61 aug-ccpVDZ,58,59,62 cc-pVTZ,58,59 aug-cc-pVTZ,58,59 aug-cc-pVTZJ,63,64 EPR-III,65 and pcJ-2.66 As PBE0, PBE, and BHandH are usually applied in coupling constant studies involving hydrogen and carbon nuclei,8,51−53,67−69 we evaluated the NJC accuracy using these functionals. For the basis set D95 and 6-311++G**, we also evaluated the performance of the B3LYP,70,71 CAMB3LYP,72 and OPBE73 functionals.

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(1) Bifulco, G.; Dambruoso, P.; Gomez-Paloma, L.; Riccio, R. Determination of Relative Configuration in Organic Compounds by NMR Spectroscopy and Computational Methods. Chem. Rev. 2007, 107, 3744−3779. (2) Tormena, C. F. Conformational analysis of small molecules: NMR and quantum mechanics calculations. Prog. Nucl. Magn. Reson. Spectrosc. 2016, 96, 73−88. (3) Contreras, R. H. High Resolution NMR Spectroscopy Understanding Molecules and their Electronic Structures; Contreras, R. H., Ed.; Elsevier: Oxford, 2013; Vol. 3. (4) Cormanich, R. A.; Moreira, M. A.; Freitas, M. P.; Ramalho, T. C.; Anconi, C. P. A.; Rittner, R.; Contreras, R. H.; Tormena, C. F. 1hJFH coupling in 2-fluorophenol revisited: Is intramolecular hydrogen bond responsible for this long-range coupling? Magn. Reson. Chem. 2011, 49, 763−767. (5) Palermo, G.; Riccio, R.; Bifulco, G. Effect of Electronegative Substituents and Angular Dependence on the Heteronuclear Spin− Spin Coupling Constant 3JC−H: An Empirical Prediction Equation Derived by Density Functional Theory Calculations. J. Org. Chem. 2010, 75, 1982−1991. (6) Favaro, D. C.; Contreras, R. H.; Tormena, C. F. Unusual ThroughSpace, TS, Pathway for the Transmission of JFHf Coupling: 2Fluorobenzaldehyde Study Case. J. Phys. Chem. A 2013, 117, 7939− 7945. (7) Schuquel, I. T. A.; Ducati, L. C.; Tormena, C. F.; de Freitas, M. P.; de Kowalewski, D. G.; Rittner, R. 13C NMR: nJCH and 1JCC scalar spin− spin coupling constants (SSCCs) for some 3-monosubstituted 2methylpropenes. J. Mol. Struct. 2014, 1068, 170−175. (8) Barbosa, T. M.; Viesser, R. V.; Martins, L. G.; Rittner, R.; Tormena, C. F. The Antagonist Effect of Nitrogen Lone Pair: 3JHF versus 5JHF. ChemPhysChem 2018, 19, 1358−1362. (9) Alabugin, I. V. Stereoelectronic Effects: A Bridge Between Structure and Reactivity; John Wiley & Sons: Chichester, 2016; pp 342−355. (10) Provasi, P. F.; Sauer, S. P. A. Analysis of isotope effects in NMR one-bond indirect nuclear spin−spin coupling constants in terms of localized molecular orbitals. Phys. Chem. Chem. Phys. 2009, 11, 3987− 3995. (11) Silla, J. M.; Freitas, M. P. The mutual effect of a carbonyl polar bond and an endocyclic oxygen on the 1JC-F coupling constant of fluorinated six-membered rings. Magn. Reson. Chem. 2017, 55, 1079− 1083. (12) Kleinpeter, E.; Koch, A.; Pihlaja, K. Application of 1J(C,H) coupling constants in conformational analysis. Tetrahedron 2005, 61, 7349−7358. (13) Wolfe, S.; Pinto, B. M.; Varma, V.; Leung, R. Y. N. The Perlin Effect: bond lengths, bond strengths, and the origins of stereoelectronic effects upon one-bond C−H coupling constants. Can. J. Chem. 1990, 68, 1051−1062. (14) Juaristi, E.; Cuevas, G. Manifestations of Stereoelectronic Interactions in 1JC−H One-Bond Coupling Constants. Acc. Chem. Res. 2007, 40, 961−970. (15) Perlin, A. S.; Casu, B. Carbon-13 and proton magnetic resonance spectra of D-glucose-13C. Tetrahedron Lett. 1969, 10, 2921−2924. (16) Salome, K. S.; Tormena, C. F. Revisiting the Long-Range Perlin Effect in a Conformationally Constrained Oxocane. J. Org. Chem. 2018, 83, 10501−10504. (17) Cuevas, G.; Martínez-Mayorga, K.; Fernández-Alonso, M. d. C.; Jiménez-Barbero, J.; Perrin, C. L.; Juaristi, E.; López-Mora, N. The Origin of One-Bond C-H Coupling Constants in OCH Fragments: Not Primarily nO→ Delocalization. Angew. Chem., Int. Ed. 2005, 44, 2360− 2364. (18) Vizioli, C.; de Azua, M. C. R.; Giribet, C. G.; Contreras, R. H.; Turi, L.; Dannenberg, J. J.; Rae, I. D.; Weigold, J. A.; Malagoli, M. Proximity Effects on Nuclear Spin-Spin Coupling Constants. 1. J(CH)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03035. Experimental 1JCHf coupling; theoretical 1JCHf coupling and its terms; summary of the main (FC) contributions for benzaldehyde and o-chlorobenzaldehyde; and molecular properties of benzaldehyde derivatives (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Angelita Nepel: 0000-0002-7426-0927 Renan V. Viesser: 0000-0002-4176-6067 Cláudio F. Tormena: 0000-0002-1508-0694 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge São Paulo Research Foundation (FAPESP, grant #2015/08541-6) for the financial support for this research as well as for the fellowship to R.V.V. (grant #2017/ 20890-1). They also acknowledge the CAPES for a scholarship to A.N.; CNPq for a fellowship to C.F.T.; National Laboratory for Scientific Computing (SDumont) for providing computa1501


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