CAr–H···O Hydrogen Bonds in Substituted Isobenzofuranone

Article pubs.acs.org/JPCA

CAr−H···O Hydrogen Bonds in Substituted Isobenzofuranone Derivatives: Geometric, Topological, and NMR Characterization Mark V. Sigalov,*,† Evgeniya P. Doronina,‡ and Valery F. Sidorkin*,‡ †

Department of Chemistry, Ben-Gurion University of the Negev, 84105 Beer-Sheva, Israel Irkutsk Institute of Chemistry, Siberian Division of RAS, Favorsky st. 1, 664033 Irkutsk, Russia

‡

S Supporting Information *

ABSTRACT: Substituted isobenzofuranone derivatives 1a− 3a and bindone 4 are characterized by the presence of an intramolecular CAr−H···O hydrogen bond in the crystal (Xray), solution (1H NMR and specific and nonspecific IEFPCM solvation model combined with MP2 and B3LYP methods), and gas (MP2 and B3LYP) phases. According to geometric and AIM criteria, the CAr−H···O interaction weakens in 1a−3a (independent of substituent nature) and in 4 with the change in media in the following order: gas phase > CHCl3 solution > DMSO solution > crystal. The maximum value of hydrogen bond energy is 4.6 kcal/mol for 1a−3a and 5.6 kcal/mol for 4. Both in crystals and in solutions, hydrogen bond strength increases in the order 1a < 2a < 3a with the rising electronegativity of the ring substituents (H < OMe < Cl). The best method for calculating 1H NMR chemical shifts (δcalcd − δexpl < 0.7 ppm) of hydrogen bonded and nonbonded protons in 1a−3a and 1b−3b (isomers without hydrogen bonds) is the GIAO method at the B3LYP level with the 6-31G** and 6-311G** basis sets. For the C−H moiety involved in the hydrogen bond, the increase of the spin−spin coupling constant 1J(13C−1H) by about 7.5 Hz is in good agreement with calculations for C−H bond shortening and for blue shifts of C−H stretching vibrations (by 55−75 cm−1).

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INTRODUCTION In the last three decades, the C−H···O hydrogen bond has attracted increased experimental and theoretical interest because of its important roles in the molecular recognition process, the binding of biologically important molecules, and the determination of molecular structure and conformation. Crystal lattice structures, for example, are consolidated by weak, intermolecular C−H···O hydrogen bonds (with energies lower than 1.5 kcal/mol), which have been intensively investigated in myriad publications over the last five years.1 In solution, these weak bonds still remain significant, and the number of examples of their investigation is also large. Spectroscopic and computational investigations of liquid alkoxy benzaldehydes and fluorobenzaldehydes2 indicated that dimeric complexes were formed by C−H···O hydrogen bonds with C2− H and carbonyl oxygen participation. The calculations predict C−H bond length shortening in some dimer structures relative to the monomer, which leads to a blue shift in C−H stretching vibrations. However, experimental IR spectroscopic data obtained for polyfluorinated benzenes as proton donors show red shifts of C−H stretching vibrations upon hydrogen bonded complex formation with various proton acceptors.3 In addition, other physicochemical properties, such as the much lower volatility of cyclic γ-butyrolactone vs that of linear methyl propionate, was explained4 by dimerization of the former compound due to the strong dipole−dipole interaction and the © 2012 American Chemical Society

C−H···O hydrogen bond (secondary effect). The relative strengths of C−H···OC and N−H···OC bonds were evaluated as 1:5 (DFT calculations) for the dimers and trimers of N-methylacetamide.5 The difference in binding energies was shown to be only 5.4 kcal/mol (4.8 kcal/mol after BSSE correction). In contrast, intramolecular C−H···O hydrogen bonds of aromatic C−H donors are not as widespread. Most of the CAr− H···O intramolecular hydrogen bonds reported are those in which the participating C−H and O−C fragments are fixed in rigid planar configurations, like i−iv as follows:6 Very few studies of CAr−H···O (both inter- and intramolecular) hydrogen bond energy estimation have been published. Hydrogen bonds that form in peptides were studied ab initio by modeling and using MP2 calculations in which water was the proton acceptor and benzene, phenol, indole, and imidazole were proton donors.7 It was shown that only C− H···O bonds involving indole and imidazole are strong enough (2.1−2.3 kcal/mol) to contribute significantly to protein structure. The N-protonation of imidazole drastically increases the energy (up to 11.3 kcal/mol) of its C−H···O hydrogen bonds. A very significant positive charge effect of 10.0 kcal/mol Received: April 25, 2012 Revised: June 13, 2012 Published: June 26, 2012 7718

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was added in portions over 30 min at room temperature to a solution of corresponding substituted phthalic dianhydride (10.14 mmol) in toluene (5 mL). After stirring for 3 h at room temperature and heating at 85 °C for 15 min, the solvent was removed under reduced pressure. The crude yellow solid obtained was purified by column chromatography on silica gel (CH2Cl2 as eluent). 4,5-Dimethoxy- and 4,5-dichlorophthalic anhydrides were prepared according to refs 12 and 13. (E)-Ethyl 2-(5,6-Dimethoxy-3-oxoisobenzofuran-1(3H)ylidene)acetate (2a). Yield 83%; mp 168−170 °C. 1H NMR, 500.13 MHz, CDCl3, δ, ppm: 8.65s, 1H, H-7; 7.31s, 1H, H-4; 6.04s, 1H, H-8; 4.27q, J = 7.15 Hz, 2H, OCH2; 4.06s, 3H; 3.99s, 3H, OCH3; 1.35t, 3H, CH3. 13C NMR, 125.76 MHz, CDCl3, δ, ppm: 165.85, 165.57, 158.02, 155.01, 153.43, 129.88, 119.50, 109.32, 105.57, 100.44, 61.06, 56.80, 56.48, 14.55. Anal. Found: C, 60.54; H 5.16. C14H14O6. Calcd: C, 60.43; H, 5.07. (Z)-Ethyl 2-(5,6-Dimethoxy-3-oxoisobenzofuran-1(3H)ylidene)acetate (2b). Yield 9%; mp 212−214 °C. 1H NMR, 500.13 MHz, CDCl3, δ, ppm: 7.32s, 1H, H-4; 7.07s, 1H, H-7; 5.75s, 1H, H-8; 4.30q, J = 7.15 Hz, 2H, OCH2; 4.01s, 3H, and 3.98s, 3H, OCH3; 1.35t, 3H, CH3. 13C NMR, 125.76 MHz, CDCl3, δ, ppm: 165.82, 163.88, 155.42, 154.34, 153.36, 133.28, 117.84, 106.07, 101.97, 94.63, 60.87, 56.61, 14.32. (E)-Ethyl 2-(5,6-Dichloro-3-oxoisobenzofuran-1(3H)ylidene)acetate (3a). Yield 81%; mp 176−177 °C. 1H NMR, 500.13 MHz, CDCl3, δ, ppm: 9.25s, 1H, H-7; 8.03s, 1H, H-4; 6.18s, 1H, H-8; 4.30q, J = 7.15 Hz, 2H, OCH2; 1.37t, 3H, CH3. 13 C NMR, 125.76 MHz, CDCl3, δ, ppm: 165.22, 163.62, 155.90, 140.48, 137.58, 135.02, 130.06, 126.73, 103.84, 61.27, 14.17. Anal. Found: C, 50.34; H 2.76; C12H8Cl2O4. Calcd: C, 50.20; H, 2.81. (Z)-Ethyl 2-(5,6-Dichloro-3-oxoisobenzofuran-1(3H)ylidene)acetate (3b). Yield 13%; mp 185−187 °C. 1H NMR, 500.13 MHz, CDCl3, δ, ppm: 8.04s, 1H, H-4; 7.84s, 1H, H-7; 0.86s, 1H, H-8; 4.30q, J = 7.15 Hz, 2H, OCH2; 1.37t, 3H, CH3. 13 C NMR, 125.76 MHz, CDCl3, δ, ppm: 163.45, 163.01, 151.98, 140.45, 137.93, 137.48, 127.54, 124.15, 123.02, 97.54, 61.21, 14.20.

was found for the interaction of dimethyl ether with the Nmethyl-pyridinium cation, which, for the neutral pyridine donor, was 1 order of magnitude less.8 Analysis of the data in the Cambridge Structural Database showed that aromatic C−H donors have the ability to form bifurcated hydrogen bonds with the participation of one acceptor oxygen atom and two adjacent aromatic hydrogen atoms. Corresponding MP2 calculations confirmed that the stabilization energy of such an interaction was larger than that for usual linear hydrogen bonds.9 Recently, CAr−H···O intramolecular hydrogen bonding in the (E)-2-(3-oxoisobenzo-furan-1(3H)-ylidene) acetate series (1) and its naphthyl analogues were studied experimentally by X-ray and NMR methods and theoretically by DFT calculations.10 The present work, carried out as a logical extension of a prior paper,10 systematically and thoroughly addresses the issue of CAr−H···O energy. The goal of the present work is to use X-ray and quantum mechanical methods, supported by NMR and topological AIM analyses, to study the influence of the substituent X on C−H···O hydrogen bond geometry and energy in compounds 1a−3a in their crystalline, solution, and gas phases.

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COMPUTATIONAL DETAILS The calculations for 1−4 were conducted by 6-311G**-based B3LYP and MP2 methods using the Gaussian 03 suite.14 The locations of the molecules at the minima of potential energy surfaces were confirmed by positive values of corresponding Hessians. The effect of the polar medium on the 1−4 structures was considered in terms of both nonspecific solvation (polarized continuum IEF-PCM15) and specific solvation (supermolecule, SM, i.e., interaction of two solvent molecules with one molecule of 1−4; in fact, the structure of the first solvate shell is undoubtedly more complex). The Wiberg bond index, WBI16, indicating bond toughness was calculated using the HF/6-311G(d,p) method. The NMR chemical shifts δcalcd 1H (H-4; H-7; H-8) for 1−3 in CHCl3 and DMSO solutions were calculated relative to TMS by GIAO HF, B3LYP, and MP2 methods in 6-31G**, 6-311G**, 6-311+ +G**, and cc-pVTZ basis sets (δcalcd 1H = σcalcd(TMS) − σcalcd(aromatic proton), where σ is the shielding constant. The AIM analysis17 of electron distribution ρ(r) in 1−4 calculated by MP2(full)/6-311++G** was carried out using the MORPHY 1.0 program.18

The X-ray measurements were conducted only for E-isomers 2a and 3a. For the sake of comparison, the crystal structures of the parent phthalide 1a10 and of its close analogue bindone 4 with firmly fixed orientation of C−H and OC bonds11 were considered.

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EXPERIMENTAL SECTION Synthesis. Previously unknown derivatives 2 and 3 were prepared according to a published procedure.10 Ethyl (triphenylphosphoranylidene)acetate (3.54 g, 10.14 mmol) 7719

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H7···O10C9 contact (especially for isolated molecules, Table 1). Nevertheless, according to dO10···H7 values and in terms of the IEF-PCM model, compared with MP2, the B3LYP method substantially underestimates O10···H7 bonding both in low polar (CHCl3) and in high polar (DMSO) solutions. When isolated molecules 1a−3a and 4 pass into solution (independent of solvation model and of geometry optimization method) and then into the crystal state, the interaction O10···H7 weakens in the order: dO10···H7 (gas) < dO10···H7 (solution) < dO10···H7 (crystal). Moreover, the C7−H7···O10 bond is stronger in the weakly polar chloroform than in the highly polar dimethylsulfoxide: dO10···H7 (CHCl3) < dO10···H7 (DMSO). Although this order of hydrogen bonding strength appears reliable, one should be aware that difficulties exist in performing X-ray measurements of C−H bond lengths.21 The influence of environment is much weaker for most of the valence bonds than for the hydrogen bond O10···H7 (see Table 1 and Figures 1 and 2). In the specific solvation models of 1a−3a and 4 (DMSO solution), the absolute values of energy of 1:2 complex formation ΔE (Figure 4) do not exceed 7.6 kcal/mol (with ZPE correction). In this model, the carbonyl oxygen O10 of 1a−3a and 4 in DMSO solution participates not only in the intramolecular hydrogen bond with H7 but also in intermolecular bonds with methyl hydrogens of the DMSO molecule (Figure 4; Table S1, Supporting Information). These competitive, specific interactions, together with nonspecific, electrostatic ones (vide supra), help to weaken the O10···H7

RESULTS AND DISCUSSION Electronic and Spatial Structures of Phthalides 1−3 and Bindone 4 in Crystals, DMSO Solution, and Their Isolated State. According to geometric criteria,19 the regions of weak and medium strength hydrogen bonds in the generic molecule A−H···B (Scheme 1; C−H···O used as an example) Scheme 1. Geometric Parameters of C−H···OC Hydrogen Bond

are determined by the following parameters: R = 2.5−3.2 Å, r = 1.5−2.2 Å, θ = 130−180° (medium bonds) and R = 3.2−4 Å, r = 2.2−3.2 Å, θ = 90−150° (weak bonds; for C−H···OC, contact φ = 120−140°).20 Values of the interatomic distances for C7···O10 (dC7···O10) and O10···H7 (dO10···H7) and of valence angles ∠C7H7O10 and ∠C9O10H7 (Table 1) indicate that, in compounds 1a−3a and 4, the intramolecular hydrogen bond C7−H7···O10C9 exists not only in the crystals but also in the solution and gas phases. Notably, a good quantitative agreement was found between the B3LYP- and MP2-calculated geometric parameters of C7−

Table 1. Geometric Parameters of the C−H···O Hydrogen Bond in 1a−3a and 4 in Gas Phase (Isolated Molecule), in CHCl3 and DMSO Solutions, and in Crystals medium gas

method B3LYP/6-311G**

MP2/6-311G**

CHCl3 solution (ε = 4.9)

IEF-PCM B3LYP/6-311G**

IEF-PCM MP2/6-311G**

DMSO solution (ε = 46.7)

IEF-PCM B3LYP/6-311G**

IEF-PCM MP2/6-311G**

SM + IEF-PCM B3LYP/6-311G**

crystal

X-ray

7720

cmpd

dC7···O10

dO10···H7

∠C7H7O10

∠C9O10H7

1a 2a 3a 4 1a 2a 3a 1a 2a 3a 4 1a 2a 3a 1a 2a 3a 4 1a 2a 3a 1a·2DMSO 2a·2DMSO 3a·2DMSO 4·2DMSO 1a 2a 3a 4

2.972 2.962 2.944 2.938 2.964 2.948 2.932 2.993 2.983 2.957 2.942 2.975 2.957 2.939 3.000 2.992 2.962 2.943 2.979 2.960 2.942 3.006 3.002 2.971 2.946 2.957 2.961 2.898 2.955

2.126 2.101 2.107 2.026 2.128 2.094 2.104 2.153 2.126 2.121 2.032 2.142 2.107 2.112 2.163 2.136 2.126 2.034 2.148 2.113 2.116 2.166 2.148 2.133 2.039 2.236 2.215 2.175 2.182

133.3 134.9 132.4 140.2 132.1 134.0 131.2 132.8 134.6 132.3 140.1 131.9 133.6 131.2 132.6 134.5 132.3 140.1 131.8 133.4 131.2 132.8 134.3 132.5 139.8 133.7 136.8 133.2 135.0

121.5 120.9 122.6 116.3 122.4 121.7 123.5 121.3 120.6 122.4 116.4 122.4 121.9 123.5 121.2 120.5 122.2 116.5 122.5 122.0 123.4 121.1 119.8 121.6 116.8 120.5 118.3 121.3 111.5

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their lengths, the hydrogen bonds in the former case are notably stronger than in the latter. It should be emphasized that, according to their positive ΔG values, the complexes 1a·2DMSO−3a·2DMSO and 4·2DMSO are unstable upon return to normal conditions (298.15 K, 1 atm) (Figure 4). Therefore, the analysis of solvent influence on the structures 1a−3a and 4 at room temperature may be carried out in terms of the nonspecific solvation (PCM) model. The presence of an O10···H7 hydrogen bond in phthalides 1a−3a and in bindone 4 was proven not only by geometric criteria but also by quantum topological standards. In fact, in their internuclear area O10···H7 (see molecular graphs in Figures 3 and Tables 2, S1, and S2), the AIM method was used to find the bond critical point [BCP (3,−1)] and to identify the formation of the corresponding seven-membered heterocycle H7−C7CCCC9O10 as indicated by the ring critical point [RCP (3,+1)]. According to the electron density ρ(r) values and their corresponding Laplacian ∇2ρ(r), the positive electron energy Ee(r), and the ∇2ρ(r) in BCP (O10···H7), the hydrogen bond (O10···H7) in the molecules 1a−3a and 4 in all media is weak (1−4 kcal/mol) and possesses ionic nature (Table 2; the results of AIM analysis for MP2-optimized 1a−3a geometries are given in Table S2, Supporting Information).22 For the crystal phase, this conclusion is supported by a quantitative evaluation of the energy EHB of the O10···H7 interaction, which, for an AIM analysis, can be carried out using the equation

Figure 1. Selected experimental (X-ray) and calculated B3LYP/6311G** (gas phase and DMSO solution) distances (Å) for phthalide 2a. C7−C10 1.393(2) (1.399, 1.398); C10−C11 1.390(2) (1.390, 1.393); C10−C1 1.471(3) (1.466, 1.463); C1−C8 1.333(3) (1.350, 1.348); C8−C9 1.464(2) (1.462, 1.464); C9−O10 1.208(2) (1.216, 1.217).

E HB = Ve/2

(1)

where Ve is the density of potential energy in BCP (O10···H7). Equation 1 was suggested and approved for intermolecular hydrogen-bonded complexes23 and then applied to those complexes24 and to other kinds of contacts.25 Moreover, the EHB values obtained with eq 1 are reliable and agree well with the estimations of O10···H7 bond energy in the gas phase according to the relative stabilities of the a and b isomers. Nevertheless, in light of the correlation character of eq 1, the EHB values found for intramolecular hydrogen bonded complexes 1a−3a and 4 must be treated carefully. It is noteworthy that, strictly according to geometric criteria (see values of dC7···O10, dO10···H7, and ∠C7H7O10 in Table 1), O10···H7 hydrogen bond strengths in 1a−3a and 4 (in any medium) may all be assigned intermediate values (4 − 15 kcal/ mol). One of the main factors governing the strength of the C− H···O hydrogen bond is C−H bond acidity.20 To estimate bond acidity, we calculated WBI16 of the C7−H7 bond without interaction with the carbonyl oxygen, i.e., in 1b−3b isomers. With the increasing electronegativity of X (H < MeO < Cl) and independent of optimization method, the WBI values of the C7−H7 bond decrease both in gas phase and in solution (PCM model), i.e., its proton donor ability increases (the WBI values for B3LYP/6-311**-optimized geometries are given in parentheses):

Figure 2. Selected experimental (X-ray) and calculated B3LYP/6311G** (gas phase and DMSO solution) distances (Å) for phthalide 3a. C7−C10 1.385(2) (1.393, 1.392); C10−C11 1.390(2) (1.397, 1.397); C10−C1 1.468(3) (1.470, 1.468); C1−C8 1.329(3) (1.346, 1.345); C8−C9 1.465(2) (1.467, 1.467); C9−O10 1.207(2) (1.214, 1.216).

1b (0.931) < 2b (0.926) < 3b (0.921)

contact in solutions of 1a−3a and 4 compared with that in isolated molecules (Tables 1 and 2). Most probably, the observed inequality [dO10···H7 (gas) < dO10···H7 (crystal)] is the result of participation of the fragment C7−H7···O10C9 and its nearest neighbors via short, intramolecular contacts within the crystals of 1a−3a and 4 (see Supporting Information). Note that the DMSO molecule acts as both hydrogen donor and acceptor for each of the hydrogen bonds with 1a−3a and 4 (Figure 4; Table S1, Supporting Information). On the basis of

(2)

Therefore, from 1a to 3a, H-bond strength is expected to rise. Moreover, according to the values of dO10···H7 and EHB, that order of H-bond strength (1a < 2a < 3a) is observed in solutions (chloroform, DMSO) and in crystals of 1a−3a (see Tables 1 and 2). The effect of X on the relatively strong C7−H7 bond is weak [see WBI values in eq 2], and thus, the differences in dO10···H7 and EHB values between phthalides 1a−3a are small (Tables 1 and 2). The order of dO10···H7 and EHB changes in 7721

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Figure 3. Optimized geometries and molecular graphs [MP2(full)/6-311++G**] of 1a−3a and 4 in gas phase. Filled squares denote BCPs (3,−1), open circles RCPs (3,+1).

Figure 4. Optimized geometries and energies of complexation of 1a−3a and 4 with two DMSO molecules.

isolated molecules is different: 1a < 3a < 2a. In this case, however, the difference in these values for 2a and 3a is insufficient.26 The increased strength of C7−H7···O10 in

bindone 4 compared with its structural analogue 1a may be explained by the fixed mutual orientation of C−H and OC fragments in the first molecule, which is favored over the 7722

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Table 2. Properties of the BCP (O10···H7) and Hydrogen Bond O10···H7 Energies EHB of 1a−3a and 4 in Gas Phase, in CHCl3 and DMSO Solutions (B3LYP/6-311G** Geometry Optimization), and in Crystals medium gas

CHCl3 solution (ε = 4.9)

DMSO solution (ε = 46.7)

crystal

cmpd

ρ(r)a

∇2ρ(r)a

Ee(r)a

EHBb

−ΔEc

1a 2a 3a 4 1a 2a 3a 4 1a 2a 3a 4 1a 2a 3a 4

0.019 0.020 0.020 0.024 0.018 0.019 0.019 0.024 0.018 0.019 0.019 0.023 0.016 0.016 0.018 0.018

0.073 0.077 0.077 0.091 0.069 0.072 0.074 0.089 0.067 0.070 0.073 0.089 0.063 0.065 0.073 0.069

0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.003 0.002

4.33 4.58 4.52 5.59 4.05 4.30 4.35 5.51 3.96 4.19 4.29 5.48 3.45 3.57 4.00 3.89

3.14 4. 14 3.70

shortening X−H bond (attraction of H to X) dominates the opposite, force-lengthening X−H bond (attraction of H to Y). The question as to whether the conditions for forming improper intermolecular hydrogen bonds can be applied to intramolecular hydrogen bonds remains unanswered. A possible reason for this outcome may be the marked difficulties in choosing the model hydrogen bond donor and acceptor groups and the need to account for the steric factor when forming intramolecular hydrogen-bonded complexes. However, in agreement with the results of ref 27a, one can suppose that the formation of a blue-shifting C7−H7···O10 bond in 1a−3a is favored by the relatively low polarity of its donor component (C7−H7) and by the relatively weak proton accepting ability of carbonyl oxygen O10. NMR Spectra. The 1H and 13C NMR spectral data for the compounds 1−3 (series a and b) recorded in CDCl3 and DMSO-d6 are collated in Table 3 together with the calculated (B3LYP/6-311G**) 1H chemical shifts. As expected, the C−H···O hydrogen bonds in the a series strongly affected the NMR chemical shifts of H-7 and C-7, which, compared with their b series counterparts, were shifted downfield by 1.3−1.6 ppm and 7.0−7.3 ppm, respectively. The change of solvent from CDCl3 to DMSO-d6 for the 1a−3a series resulted in an upfield shift in H-7. Such behavior by H-7 probably stemmed from the weakening and lengthening C O···H bond, which, as it is shown above, may be due to the effect of solvent reaction field. The analysis of calculated 1H chemical shifts (Tables 3 and S3) shows the following: (1) The values of δcalcd have low sensitivity to the geometry optimization method (for instance, the difference between δ calcd 1 H B3LYP/6-311G**//IEF-PCM B3LYP/6-311G** and B3LYP/6-311G**//IEF-PCM MP2/6-311G** do not exceed 0.2 ppm). (2) In general, the best agreement of δexptl and δcalcd, both for hydrogen-bonded H-7 and for non-bonded H-4 and H-8 (δcalcd − δexptl < 0.7 ppm), was achieved using the B3LYP level with the 6-31G** and 6-311G** basis sets. However, it is worth noting that the accuracies of δ(H4) and δ(H-8) calculations were higher than that of δ(H7); see Table S3, Supporting Information. (3) The worst agreement for δ(H-7) was obtained with the HF and MP2 methods independently of the basic set (δcalcd − δexptl > 1 ppm). This finding was unexpected regarding the MP2 level because, according to data from

a

MP2(full)/6-311++G**-calculated electron density [ρ(r), a.u.], its Laplacian [∇2ρ(r), a.u.], and electron energy density [Ee(r), a.u.] in BCP (O10···H7). bThe H-bond energy [EHB, kcal/mol], estimated by eq 1. cThe O10···H7 H-bond energy [ΔE, kcal/mol] in the gas phase evaluated as the difference between the total energies of the a and b isomers.

second molecule for H-bond formation. For example, the valence angle ∠C7H7O10 in 4 is closer than that in 1a to the optimum value of 180°. A comparison of the dC7−H7 bond lengths in 1a−3a and 1b− 3b shows that C7−H7···O10 H-bond formation is accompanied by a shortening of the valence C7−H7 bond (by 0.002−0.004 Å in gas phase and chloroform solution and by 0.0035−0.0075 Å in DMSO solution). This finding confirms that C7−H7···O10 interaction in the phthalides 1a−3a belongs to improper blueshifting hydrogen bonds.27 In agreement with this, B3LYP/6311G**-calculated vibrational frequencies of the C7−H7 bond in 1a−3a are in the range of 3239−3245 cm−1, whereas those in 1b−3b are in the range of 3166−3217 cm−1 (Table S4, Supporting Information). The universal explanation of the formation of improper intermolecular hydrogen bonds X− H···Y was suggested in ref 27a. Basically, they contend that, given hydrogen donor X−H and acceptor Y, the force-

Table 3. 1H and 13C NMR Spectral Data for 1−3 (a and b series) in CDCl3 and DMSO-d6 Solutions; Calculated Values δcalcd 1H (GIAO B3LYP/6-311G**//B3LYP/6-311G**) Are Given in Brackets entry

X

1a

H

2a

MeO

3a

Cl

1b

H

2b

MeO

3b

Cl

solvent CDCl3 DMSO-d6 CDCl3 DMSO-d6 CDCl3 DMSO-d6 CDCl3 DMSO-d6 CDCl3 DMSO-d6 CDCl3 DMSO-d6

H-4 8.01 8.12 7.31 7.46 8.03 8.38 8.01 8.11 7.32 7.38 8.04 8.32

[8.23] [8.29] [7.22] [7.31] [8.08] [8.15] [8.21] [8.24] [7.18] [7.29] [8.04] [8.16]

H-7 9.09 8.94 8.65 8.48 9.25 9.07 7.79 8.34 7.07 7.80 7.84 8.62

H-8

[9.77] [9.70] [9.00] [8.96] [9.80] [9.75] [8.02] [8.12] [7.02] [7.18] [7.88] [8.07]

6.19 6.33 6.04 6.09 6.18 6.35 5.92 6.51 5.75 6.27 5.86 6.47 7723

[6.23] [6.27] [6.06] [6.09] [6.25] [6.29] [5.94] [6.03] [5.68] [5.86] [5.83] [6.02]

C-4 (1JC−H) 125.3 125.4 105.6 106.8 126.7 127.7 126.0 125.5 106.1 106.5 127.5 127.7

(167.0) (168.9) (164.8) (166.3) (173.9) (175.9)

(165.3) (166.3) (172.7) (174.7)

C-7 (1JC−H) 128.2 127.3 109.3 109.0 130.1 129.1 121.2 122.5 102.0 104.7 123.0 125.1

(172.7) (172.7) (168.9) (168.2) (178.3) (177.2) (165.1) (167.0) (162.2) (167.0) (170.8) (174.7)

C-8 (1JC−H) 102.4 (163.1) 101.8 (165.1) 100.8 (163.4) 100.4 (163.6) 103.8 (164.3) 103.6 (165.2) 96.0 (163.1) 95.9 (167.0) 94.6 (163.8) 94.9 (166.7) 97.5 (165.1) 97.9 (168.9)

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the literature,28 calculation of the chemical shift of a hydrogen-bonded proton requires methods that include electron correlation. In an earlier paper11b focused on bindone 4, we separated the effect of the hydrogen bond (0.6 ppm) from the overall deshielding effects, including steric, anisotropic, and electric field effects of the CO group on the 1H NMR chemical shift of the participating proton (1.8 ppm). The existence of two isomers for the studied compounds allows the order of hydrogen bond strength to be approximately evaluated by analyzing the H-7 chemical shift difference between isomers a and b and comparing the findings with the similar values obtained for H-4 (Table 4).

was considerably higher (5.5−5.6 kcal/mol). The effects of media and solvent on the hydrogen bond were investigated for crystalline phase, solution (chloroform and DMSO), and gas phase, and it was shown that the C−H···O interaction weakened with the change of media in the following order: (gas phase) > (solution) > (crystal). In DMSO solution, the hydrogen bond was weaker than that in chloroform. Carbonyl oxygens and activated aromatic hydrogens were able to form intermolecular hydrogen bonds with DMSO, which acted both as hydrogen bond donor and acceptor. However, these interactions were relatively weak (1.3−1.8 kcal/mol and 2.5− 3.5 kcal/mol, respectively), and the complexes of 1−3 with DMSO were not stable at ambient temperature based on the positive ΔG values of their formation. Therefore, the solvent effect on the hydrogen bond strength (and on 1H NMR chemical shifts) may be explained in terms of nonspecific interactions (PCM model) without taking into account the complexation. By comparison of the calculated δcalcd (GIAOHF, -MP2, and -B3LYP with 6-31G**, 6-311G**, 6-311+ +G**, and cc-pVTZ basis sets) and experimental δexptl 1H chemical shifts both for hydrogen bonded H-7 and for nonbonded H-4 and H-8, in 1a−3a and 1b−3b, the advantage of GIAO- B3LYP with 6-31G** and 6-311G** basis sets was established (δcalcd − δexptl < 0.7 ppm). The failure of GIAOMP2 method at δexpl H-7 description (δcalcd − δexptl > 1 ppm) is unexpected. C−H···O H-bond formation was accompanied by a shortening of the valence C−H bond, reflected in the blue shifts of vibrational frequencies (by 55−75 cm−1) calculated for this bond and in the corresponding increase of one-bond 13 C−1H spin−spin coupling constants.

Table 4. 1H NMR Chemical Shift Differences of H-7 and H-4 between Isomers 1a−3a and 1b−3b CDCl3

DMSO

entry

Δδ(H-7)

Δδ(H-4)

Δδ(H-7)

Δδ(H-4)

1 2 3

1.30 1.58 1.41

0 −0.01 −0.01

0.60 0.68 0.45

0.01 0.08 0.06

The chemical shift values Δδ(H-4) are close to zero independently of the substituent X in CDCl3 solution (and a similarly small influence was also observed in the DMSO solution) because δ(H-4) reflects only the electronic effects, whereas, because of the changes in the C−H···O fragment geometry, Δδ(H-7) was sensitive both to the substitution and to the medium. This difference was much lower in DMSO than in CDCl3, which unambiguously indicates the weakening of the H-bond in the first case. The observed order of Δδ(H-7) changes (1a < 3a < 2a) is in agreement with AIM evaluations of hydrogen bond energy using MP2-optimized rather than B3LYP-optimized geometries of phthalides. This finding may be due to the fact that the differences in the values characterizing the strengths of the O10···H7 bond [dC7···O10, EHB, Δδ(H-7)] in 1a−3a are small, and therefore, the order of their change, depending on X, is sensitive to the evaluation methods. The one-bond 13C−1H spin−spin coupling constants deserve more attention because they reveal an unusual and interesting feature. These constants in hydrogen-bonded fragments of 1a− 3a molecules are about 6.7−7.5 Hz larger than the corresponding values in the 1b−3b series (Table 3). Usually the X−H···Y hydrogen bond formation leads to a weakening of the X−H bond and a decrease in the X−H one-bond coupling.29 The increase of one-bond spin−spin coupling mostly indicates a rise in the s-character and a shortening of the bond.30 This is in agreement with the C7−H7 bond lengths discussed above and their vibrational frequencies, and it also proves that the C−H···O bond observed in the compounds studied (and in bindone 411b) belongs to blue-shifting hydrogen bonds.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 with molecular graphs of 1a-3a and 4 in DMSO solution, Tables S1−S2 with additional data of AIM analysis for 1a−3a, Table S3 with calculated 1H NMR chemical shifts for 1a−3a and 1b−3b, Table S4 with calculated C7−H7 bond lengths and vibrational frequencies, Cartesian coordinates of calculated structures 1−3 and their 1:2 complexes with DMSO. Crystallographic data for 2a and 3a. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.V.S.); [email protected] (V.F.S.). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) A SciFinder database search shows about 8000 publications from the last five years (2007−2011) that mentioned the presence of the C−H···O hydrogen bond. (2) (a) Karger, N.; Amorim da Costa, A. M.; Ribeiro-Claro, P. J. A. J. Phys. Chem. A 1999, 103, 8672−8677. (b) Marques, M. P. M.; Amorim da Costa, A. M.; Ribeiro-Claro, P. J. A. J. Phys. Chem. A 2001, 105, 5292−5297. (c) Ribeiro-Claro, P. J. A.; Marques, M. P. M.; Amado, A. M. ChemPhysChem 2002, 3, 599−606. (3) Ventkatesan, V.; Fujii, A.; Ebata, T.; Mikami, N. J. Phys. Chem. A 2005, 109, 915−921. (4) Hesse, S.; Suhm, M. A. Phys. Chem. Chem. Phys. 2009, 11, 11157−11170.

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CONCLUSIONS Derivatives 1a−3a were used as models to demonstrate and study intramolecular CAr−H···O hydrogen bonding using X-ray structure determinations and 1H NMR spectra. Quantum mechanical calculations and the AIM approach produced hydrogen bond energy estimations that varied between 3.5 and 4.5 kcal/mol. For the rigid bindone molecule 4, that value 7724

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