On prediction of OH stretching frequencies in intramolecularly hydrogen bonded systems

Journal of Molecular Structure 1018 (2012) 8–13

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On prediction of OH stretching frequencies in intramolecularly hydrogen bonded systems Poul Erik Hansen ⇑, Jens Spanget-Larsen ⇑ Department of Science, Systems and Models, Roskilde University, Universitetsvej 1, P.O. Box 260, DK-4000 Roskilde, Denmark

a r t i c l e

i n f o

Article history: Available online 20 January 2012 Keywords: OH stretching frequencies Two-bond isotope effects OH chemical shifts DFT calculations Intramolecular hydrogen bonding

a b s t r a c t OH stretching frequencies are investigated for a series of non-tautomerizing systems with intramolecular hydrogen bonds. Effective OH stretching wavenumbers are predicted by the application of empirical correlation procedures based on the results of B3LYP/6-31G(d) theoretical calculations in the harmonic and PT2 anharmonic approximations, as well as on experimental NMR parameters, i.e., proton chemical shifts (dH) and two-bond deuterium isotope effects on 13C chemical shifts (2DCOD). The procedures are applied in a discussion of the spectra of 2,6-dihydroxy-4-methylbenzaldehyde and 8-hydroxyquinoline N-oxide. The spectrum of the former displays a broad, composite band between 3500 and 2500 cm1 which can be assigned to overlapping monomer and dimer contributions. In the latter case, the results support a reassignment of the OH stretching band of 8-hydroxyquinoline N-oxide; the reassignment is supported by correlation with the IR spectra of a series of substituted derivatives. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The OH stretching vibrations of intramolecularly hydrogen bonded systems frequently give rise to broad and complex band profiles in the infrared (IR) absorbance spectra. This can be explained by anharmonic effects, leading to coupling of the OH stretching vibration with low-frequency modes of the molecular scaffold, Fermi resonances, and Franck-Condon type vibrational progressions as described, e.g., by Matanovic and Doslic [1]. In combination with inhomogeneous line broadening, this may lead to the appearance of broad and diffuse bands covering a wide wavenumber range, and in a number of cases an OH band may not easily be identified in the observed spectra. Recently, it was shown that OH stretching wavenumbers calculated in the second-order perturbation theoretical (PT2) anharmonic approximation tend to be linearly related to those obtained within the standard harmonic analysis [2]. Moreover, an approximately linear relationship was established between the observed band centers mOH and the results of B3LYP/6-31G(d) harmonic analyses for a large number of intramolecularly hydrogen bonded systems, covering the range from weak to strong hydrogen bonding [2]. In this paper we explore the predictive potential of these results by application to some difficult cases relating to OH stretching bands in non-tautomerizing systems. ⇑ Corresponding authors. Tel.: +45 46 742431; fax: +45 46 743011 (P.E. Hansen), tel.: +45 46742710; fax: +45 46743011 (J. Spanget-Larsen). E-mail addresses: [email protected] (P.E. Hansen), [email protected] (J. SpangetLarsen). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2012.01.011

We first demonstrate the performance of the approach by application to OH stretching wavenumbers for the following series of intramolecularly hydrogen bonded, non-tautomeric compounds (Scheme 1): salicylaldehyde (1), 2-hydroxyacetophenone (2), methyl salicylate (3), salicylamide (4), 2,6-dihydroxyacetophenone (5), 2-hydroxythioacetophenone (6), methyl 2,6-dihydroxy-4methylbenzoate (7), 1,3,6-triacetyl-2,4,6-trihydroxybenzene (8), dehydracetic acid (9), 5-acetyl meldrums acid (10), 2,20 -dihydroxybenzophenone (11), anthralin (12), (o-hydroxybenzoyl) benzoylmethane enol (13), (endo,endo)-pentacyclo[5.4.0.02,6.03, 10.05,9]undecane-8,11-diol (14), usnic acid (15), and 1,8-dihydroxynaphthalene (16). In addition, we establish new correlations with proton NMR chemical shifts dH and with two-bond deuterium isotope effects on 13C NMR chemical shifts 2DCOD. We then apply the resulting correlation procedures in the discussion of some difficult cases, namely the IR spectrum of 2,6-dihydroxy-4-methylbenzaldehyde (17, Scheme 2) and that of 8-hydroxyquinoline N-oxide (18), including a number of its derivatives (19–24, Scheme 3). 2. Experimental The IR absorbance spectra of 17–20 and 22 were recorded at room temperature on a Perkin–Elmer Spectrum 2000 FT-IR spectrophotometer in the 4000–370 cm1 range as an average of 10 scans with a resolution of 1 cm1. The spectrum of 17 was measured in 0.05 mol L1 CDCl3 liquid solution (path length 0.01 cm), and at ten times dilution (path length 0.10 cm). 18, 19, 20 and 22 were measured in KBr tablets. IR data for 1–15 [2] and 16 [3] were taken from the literature.

P.E. Hansen, J. Spanget-Larsen / Journal of Molecular Structure 1018 (2012) 8–13

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Scheme 1. Structures of the compounds 1–16. The hydroxy groups considered in the present study are indicated by bold hydrogen symbols.

3. Calculations All calculations were performed with the Gaussian 09 software package [4]. Gas phase equilibrium geometries and OH stretching wavenumbers in the harmonic and PT2 anharmonic [5,6] (Freq = anharmonic [4]) approximations were computed as previously described [2] by using B3LYP density functional theory [7,8] and the 6-31G(d) basis set [4]. This level is consistent with the recommendations of Barone and coworkers [5,6] for medium to large molecular systems. The anharmonic calculation is extremely time-consuming; for this reason, the anharmonic calculation was not attempted for some of the larger species. The resulting wavenumbers are listed in Tables 1–3 (the results for 1–15 are quoted from previous work [2]. The calculations on 20 and 21 were repeated with larger basis sets for the chlorine atoms (6-31+G(d) or 6-311+G(3df) [4]), leading to a lowering of the predicted OH stretching wavenumbers by 10–20 cm1. This is a relatively marginal effect, and for the sake of consistency only results obtained with the 6-31G(d) basis set for all atoms are listed for 20 and 21 (Table 3).

4. Results and discussion 4.1. Compounds 1–16 The computed OH harmonic and anharmonic stretching wavenumbers (cm1) for this series of compounds are listed in Table 1 under the headings Harm and Anh. The table also contains observed wavenumbers mOH and wavenumbers P(Harm) and P(Anh)

Scheme 2. 2,6-dihydroxy-4-methylbenzaldehyde (17): structures of endo and exo OH-rotamers for monomer (bottom) and dimer (top).

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P.E. Hansen, J. Spanget-Larsen / Journal of Molecular Structure 1018 (2012) 8–13

PðAnhÞ ¼ 1070 þ 0:695  Anh

ð2Þ

P(Harm) and P(Anh) indicate predicted, effective values of the

mOH wavenumbers from the results of harmonic and PT2 anharmonic B3LYP/6-31G(d) analyses, respectively. These relations were derived on the basis of results for a series of about 30 intramolecularly hydrogen bonded compounds, covering the range from weak to strong hydrogen bonding [2]. The least squares standard deviations were SD = 74 cm1 for P(Harm) and SD = 89 cm1 for P(Anh) [2]. In Figs. 1 and 2 the observed wavenumbers mOH are plotted against the predicted values P(Harm) and P(Anh) for the series 1–16. The mean absolute deviations for this set of compounds are MAD = 45 cm1 for P(Harm) and MAD = 50 cm1 for P(Anh). The largest deviation between experimental and predicted wavenumber is about 200 cm1 and is observed for usnic acid (15). However, the experimental value in question (2800 cm1) reported by Forsén et al. [9] refers to a broad shoulder in the IR spectrum and a representative wavenumber is difficult to estimate. Table 1 also lists experimental proton NMR chemical shifts dH [2] and two-bond deuterium isotope effects on 13C NMR chemical shifts 2DCOD for most of the compounds. As shown in Fig. 3, there is an excellent correlation between observed mOH (cm1) and dH (ppm) values, leading to the least squares linear regression equation

PðdH Þ ¼ 4400  110:3  dH

ð3Þ 1

with regression coefficient R = 0.98 and SD = 65 cm (16 points). This relationship should have considerable predictive value for the OH stretching wavenumbers for this class of systems. Fig. 4 shows the corresponding correlation between mOH (cm1) and 2DCOD (ppm):

Pð2 DCOD Þ ¼ 3388  1146:4  2 DCOD

ð4Þ

with R = 0.91 and SD = 109 cm1 (14 points). This relationship may also be useful, but seems to be of less predictive value. 4.2. 2,6-Dihydroxy-4-methylbenzaldehyde (17) Scheme 3. Structures of the 8-hydroxyquinoline N-oxides 18–24.

predicted by the empirical correlation formulas established in our previous work [2]:

PðHarmÞ ¼ 757 þ 1:171  Harm

ð1Þ

The IR spectrum of this compound in CDCl3 liquid solution displays a complex band system in the OH stretching region, see Fig. 5, characterized by a broad and diffuse band profile between 3500 and 2500 cm1. At high concentrations a broad maximum is observed around 3300 cm1 (curve a), but dilution leads to a decrease in intensity of this feature (curve b). This indicates that the

Table 1 Calculated and observed data for the compounds 1–16: OH stretching wavenumbers (cm1) computed in the harmonic (Harm) and anharmonic (Anh) approximations, observed vibrational wavenumbers mOH (cm1) [2,3], proton NMR chemical shifts dH (ppm) [2], and two-bond deuterium isotope effects on 13C NMR chemical shifts 2DCOD (ppm) [10-16]. P(Harm), P(Anh), P(dH), and P(2DCOD) indicate OH stretching wavenumbers predicted by empirical correlation procedures (see text).

1 2 3 4 5

Salicylaldehyde 2-Hydroxyacetophenone Methyl salicylate Salicylamide 2,6-Dihydroxyacetophenone

6 7

2-Hydroxythioacetophenone Methyl 2,6-dihydroxy-4-methylbenzoate

8 9 10 11 12 13 14 15

1,3,6-Triacetyl-2,4,6-trihydroxybenzene Dehydracetic acid 5-Acetyl meldrums acid 2,20 -Dihydroxybenzophenone Anthralin (o-Hydroxybenzoyl)benzoylmethane (endo,endo)-Pentacyclo[5.4.0.02,6.03,10.05,9]undecane-8,11-diol Usnic acid

16

1,8-Dihydroxynaphthalene

Harm

Anh

mOH

dH

2

3344 3281 3387 3271 3747 3166 3107 3631 3405 2719 2849 2993 3409 3342 3297 3585 3191 3301 3666

3063 2953 3119 2914 3547 2839 2707 3421 3144 – 2209 2440 3133 3036 3017 3383 2801 3057 3444

3190 3100 3258 3070 3590 3000 2900 3457 3200 2440 2610 2740 3300 3041 3150 3378 2800 3100 3477

11.01 12.26 10.74 – 6.75 13.32 13.35 8.59 11.07 17.09 16.69 15.07 10.57 12.23 12.08 – 13.28 11.03 –

0.227 0.267 0.183 – – 0.364 0.340 – 0.179 0.722 0.801 0.576 0.220 0.248 0.268 – 0.339 0.156 –

DCOD [10] [10] [10]

[11] [12] [11] [13] [14] [15] [10] [10] [16] [10] [10]

P(Harm)

P(Anh)

P(dH)

P(2DCOD)

3159 3085 3209 3073 3631 2950 2881 3495 3230 2427 2579 2748 3235 3156 3104 3441 2980 3108 3536

3199 3122 3238 3095 3535 3043 2951 3448 3255 – 2605 2766 3247 3180 3167 3421 3017 3195 3464

3186 3048 3215 – 3655 2931 2927 3452 3179 2515 2559 2738 3234 3051 3068 – 2935 3183 –

3128 3082 3178 – – 2971 2998 – 3183 2561 2470 2728 3136 3104 3081 – 3000 3209 –

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P.E. Hansen, J. Spanget-Larsen / Journal of Molecular Structure 1018 (2012) 8–13 Table 2 Predicted OH stretching transitions for monomers and dimers of 2,6-dihydroxy-4methylbenzaldehyde (17): wavenumbers Harm (cm1) and IR intensities I (km mol1) computed in the harmonic approximation, and wavenumbers P(Harm) predicted by application of Eq. (1); exo and endo indicate the conformation of the ‘free’ non-hydrogen bonded OH group (Scheme 2; see the discussion in Section 4.2). Harm Monomer Exo Endo Dimer Exo

Endo

I

P(Harm)

3753 3257 3769 3312

54 258 39 224

3638 3057 3656 3212

3751 3453 3357 3253 3771 3471 3384 3258

65 1568 563 250 47 1341 507 248

3635 3286 3175 3052 3659 3307 3206 3058

3300 cm1 band is due to dimer (or oligomer) formation. Our calculations indicate that the most stable dimer of 17 involves intermolecular hydrogen bonding between the carbonyl group of one species and the ‘free’ hydroxy group of another, as indicated in Scheme 2. The predicted monomer and dimer OH stretching transitions are listed in Table 2 and the wavenumbers are indicated in Fig. 5. The transition due to the hydroxy group involved in intermolecular hydrogen bonding is predicted close to 3300 cm1, in excellent consistency with the position of the observed dimer band. This transition is predicted to be very intense, suggesting that even a minor concentration of dimers should give rise to a recognizable dimer band. The intramolecular hydrogen bond to the carbonyl group involved in the dimer interaction is weakened, leading to an increase of the OH stretching wavenumber by more than 100 cm1 and an increase in intensity. The resulting transition is predicted around 3200 cm1 and is thus likely to contribute to the dimer band. In this investigation we focus on stretching wavenumbers for hydrogen bonded OH groups, but it has not escaped our attention that the spectrum of 17 exhibits two peaks at 3690 and 3590 cm1 in the region characteristic for ‘free’ or weakly perturbed OH groups. The observation of two wavenumbers in this region is puzzling, since the species is expected to have only one ‘free’ hydroxy group, and neither of the peaks is easily assigned to a summation band. One explanation might be that the peaks are due to endo and exo rotamers of the ‘free’ OH group in 17, as indicated in Scheme 2. Similar rotamer-splitting of OH stretching peaks has been observed for o-alkyl phenols [17,18]. The predicted OH stretching wavenumbers for endo and exo rotamers are close to 3660 and 3640 cm1, respectively (Table 2). The predicted splitting (ca. 20 cm1) is thus much smaller than the experimental one

Fig. 1. Observed OH stretching wavenumbers mOH against wavenumbers P(Harm) predicted by application of Eq. (1) for the compounds 1–16. The line indicates perfect correlation, i.e., mOH = P(Harm).

Fig. 2. Observed OH stretching wavenumbers mOH against wavenumbers P(Anh) predicted by application of Eq. (2) for the compounds 1–16. The line indicates perfect correlation, i.e., mOH = P(Anh).

(100 cm1). Moreover, the endo isomer is predicted by our B3LYP/6-31G(d) calculations to be 2–2½ kcal mol1 higher in energy than the exo rotamer, implying that the former should constitute only a marginal component of the equilibrium mixture at room temperature. Hence, a proper understanding of the IR spectrum of 17 requires additional experimental and theoretical investigations.

Table 3 Predicted and observed data for 8-hydroxyquinoline N-oxide (8-HQNO) and a series of derivatives (Scheme 3): OH stretching wavenumbers (cm1) computed in the harmonic (Harm) and anharmonic (Anh) approximations; observed proton NMR chemical shifts (dH) [20] and two-bond deuterium isotope effects on 13C chemical shifts (2DOD) (ppm) [20]. P(Harm), P(Anh), P(dH), and P(2DOD) indicate OH stretching wavenumbers predicted by the pertinent empirical correlation procedures (Eqs. (1)–(4)).

18 19 20 21 22 23 24

8-HQNO 2-Methyl-8-HQNO 5-Chloro-8-HQNO 5,7-Dichloro-8-HQNO 5-(Phenylazo)-8-HQNO 5-Nitro-8-HQNO 5,7-Dinitro-8-HQNO

Harm

Anh

dH

2

P(Harm)

P(Anh)

P(dH)

P(2DOD)

3103 3103 3077 3012 2936 2889 2669

2648 2690 2641 2464 – 2228 1788

15.10 15.55 15.14 – 16.00 17.18 20.38

0.374 0.373 0.364 – 0.487 0.642 –

2877 2877 2846 2770 2681 2626 2368

2910 2940 2905 2782 – 2618 2313

2734 2685 2730 – 2635 2505 2152

2959 2961 2971 – 2830 2652 –

DOD

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Fig. 3. Linear regression of observed OH stretching wavenumbers mOH on NMR chemical shifts dH for the compounds 1–15. The regression equation is Eq. (3): P(dH) = 4400–110.3dH (R = 0.98, SD = 65 cm1).

Fig. 6. IR absorbance spectra of 8-hydroxyquinoline N-oxide (8-HQNO, 18) and the derivatives 19, 20, and 22 in KBr tablet. The vertical lines indicate OH stretching wavenumbers P(Harm) and P(dH) predicted by application of Eqs. (1) and (3).

4.3. 8-Hydroxyquinoline N-oxide (18) and its derivatives (19–24)

Fig. 4. Linear regression of observed OH stretching wavenumbers mOH on two-bond deuterium isotope effects on 13C NMR chemical shifts 2DCOD for the compounds 1– 15. The regression equation is Eq. (4): P(2DCOD) = 3388–1146.4  2DCOD (R = 0.91, SD = 109 cm1).

Another difficult case is that of 8-Hydroxyquinoline N-oxide (18) and its derivatives, see Fig. 6. Their spectra are characterized by broad and complex band profiles between 3000 and 2000 cm1. Dziembowska et al. [19] assigned a feature at 2330 cm1 in the spectrum of 18 to the mOH band center. Under the assumption that Eqs. (1)–(4) are applicable to this series of compounds, we arrive at a different assignment. As listed in Table 3, all four criteria predict a considerably higher wavenumber for 18 in the approximate region 2900–2700 cm1. Fig. 6 indicates mOH values P(Harm) and P(dH) predicted by Eqs. (1) and (3) for the series 18, 19, 20, and 22, suggesting a slight decrease of the wavenumber through the series, consistent with the experimental trend. The electronegative nitro substituents in 23 and 24 polarize the system, thereby strengthening the hydrogen bonding and leading to the prediction of even lower mOH values: 2600–2500 cm1 for 23 and 2300–2200 cm1 24 (Table 3). Acknowledgements The authors are indebted to Eva M. Karlsen for recording the FTIR spectra presented in this work. The authors also wish to thank Professor Teresa Dziembowska for access to the 8-hydroxyquinoline N-oxides. References

Fig. 5. OH stretching region of the IR absorbance spectrum of 2,6-dihydroxy-4methylbenzaldehyde (17). a indicates the spectrum of a 0.05 mol L1 solution in CDCl3 (path length = 0.01 cm), and b the spectrum of a ten times diluted solution (path length = 0.10 cm). The vertical bars indicate predicted OH-stretching wavenumbers for monomer (full) and dimer (dashed) configurations, see Scheme 2 and Table 2.

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