Does tautomeric equilibrium exist in ortho-nitrosonaphthols?

Does tautomeric equilibrium exist in ortho-nitrosonaphthols?

Galya Ivanova, Venelin Enchev *

Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad G Bonchev str, 1113 So®a, Bulgaria

Received 2 October 2000; in ®nal form 9 January 2001

Abstract

The structure and conformational equilibrium of the monooximes of 1,2-naphthoquinone were studied by solid and

liquid state NMR spectroscopy and non-empirical quantum-chemical calculations. According to the experimental data

and the ab initio HF/6-31G** and MP4 SDTQ)/6-31G**//6-31G** levels) calculations the compounds studied exist in

the gas phase and in solution as oxime tautomers only. The relative stabilities of the above compounds in chloroform

and dimethylsulfoxide solution are calculated within the polarizable continuum model. Solvent e�ects are found to

change the relative stability of the syn- and anti-isomers of 1,2-naphthoquinone-2-oxime. The presence of syn- and anti-

oxime isomers of 1,2-naphthoquinone-2-oxime and two rotameric forms of syn-1,2-naphthoquinone-1-oxime in solu-

tion is proved by NMR spectroscopy. Ó 2001 Elsevier Science B.V. All rights reserved.

Keywords: NMR; Solvent; Ab initio calculations; Oxime; Nitroso

1. Introduction

The nitroso±oxime tautomerism of 1-nitroso-

2-naphthol and 2-nitroso-1-naphthol has been

treated in several papers [1±8]. By means of UV [1],

IR [2], and NMR [3±5] spectroscopy it has been

shown that 1-nitroso-2-naphthol exists in solution

in oxime form only. According to X-ray data [6] in

the solid state the same compound exists as an syn-

oxime tautomer. However, the existence of both

tautomeric forms of 2-nitroso-1-naphthol in chlo-

roform [4] and dimethylsulfoxide adapted with 5%

dioxane or acetic acid [3,4] has been suggested,

based on 1H NMR data. To our knowledge there

are only two theoretical studies on the nitroso±

oxime tautomerism of two ortho isomers of the

nitrosonaphthol [7,8]. Pilipenko and Savranskii

[7] have reported that the p-electron energies of

quinone monooxime tautomers of 1-nitroso-2-

naphthol and 2-nitroso-1-naphthol, calculated by

means of the semiempirical PPP method, are lower

than those of their respective nitroso tautomers. A

more systematic theoretical study concerning the

tautomerism of ortho-nitrosonaphthols has been

reported by Krzan et al. [8]. Using HF/6-31G

ab initio calculations the authors have shown that

the nitroso tautomers of 1-nitroso-2-naphthol and

2-nitroso-1-naphthol are more stable than the ox-

ime forms by 0.5 and 0.01 kcal/mol, respectively.

However, these results are in contradiction with

the experimental data published before [1±6].

The aim of the present study is the reinvesti-

gation of the structure and relative stabilities of the

tautomeric forms of 1-nitroso-2-naphthol and 2-

nitroso-1-naphthol in the gas phase, solution non-

polar and polar solvents) and solid state by a

Chemical Physics 264 2001) 235±244

www.elsevier.nl/locate/chemphys

* Corresponding author. Fax: +359-2-700225.

E-mail address: [email protected] V. Enchev).

0301-0104/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.

PII: S0301-0104 01 )00245-2

combination of NMR and ab initio quantum-

chemical methods. The solvent e�ect on the rela-

tive stabilities of the possible tautomeric, isomeric

and rotameric forms of both nitrosonaphthols is

estimated using the polarizable continuum model

PCM).

2. Computational and spectroscopic methods

2.1. Computational details

Ab initio calculations were performed with 3-

21G [9] and 6-31G** [10,11] basis sets at Hartree±

Fock level using the GAMESS package [12]. The

geometry optimizations of the structures investi-

gated Figs. 1 and 2) were done without geomet-

rical constrains. The mean gradient threshold was

1 � 10ÿ4 hartree/Bohr. The transition structures

for tautomeric �A1 ! B1� and rotameric �A1 !A2� conversions were located at 6-31G** level.

Numerical harmonic vibrational frequency calcu-

lations were carried out to characterize the nature

of all stationary points at 6-31G** level and to

estimate the zero-point vibrational energy ZPE).

The ZPE values were scaled by 0.893 [13] to ac-

count for the overestimation of vibrational fre-

quencies. The scaled ZPE corrections were in-

cluded in the relative energy values. All transition

structures were characterized by only one imagi-

nary vibrational frequency. The force calculations

at the equilibrium structures of the selected species

yielded only real frequencies.

To further correct for electron correlation,

single-point calculations were carried out at the

MP4 SDTQ)/6-31G**//6-31G** level of theory.

To account for solvent e�ects on the relative

stabilities of selected species of the compounds

investigated, the PCM implemented in GAMESS

was used. The method computes the free energy of

solvation as the sum of electrostatic, cavitation

and van der Waals components. Speci®c compu-

tational details regarding the PCM formalism are

extensively described in reviews [14,15]. In our

investigations the electrostatic and cavitation

contributions were taken into account. The cavi-

tation component was evaluated following Pier-

ottiÕs theory [16]. It has been reported that the

solvent e�ect on the geometrical parameters is

small [17,18]. Hence for the sake of economy, the

structures investigated were not reoptimized in

solution.

Fig. 1. Oxime A and nitroso B tautomeric forms of 1-nitroso-2-naphthol 1.

236 G. Ivanova, V. Enchev / Chemical Physics 264 2001) 235±244

2.2. NMR measurements

The NMR spectra were obtained on a Bruker

Avance DRX-250 spectrometer, operating at

250.13 and 62.90 MHz for 1H and 13C, respec-

tively, using a dual 5 mm probe head. The chem-

ical shifts were referenced to tetramethylsilane

TMS). The measurements in CDCl3, DMSO-d6

and DMSO-d6 � 5% CD3COOD solutions were

carried out at ambient temperature 300 K).

The following standard Bruker library pulse

programs were used for all experiments: zg, dept,

zgig, cosydftp, invbtp, inv4lplrnd. Standard 1D

experiments with 30° pulses, 1 s relaxation delays,

16K time domain points, zero-®lled to 64K for the

protons and 32K for the carbons were performed.

Hard pulses with 90° pulse widths of 11.8 ls for

the protons and 6.4 ls for the carbons at a power

level of 3 dB below the maximum output were

used. The distortionless enhancement by polar-

ization transfer DEPT) spectra were obtained

under the same conditions as the 13C-NMR spec-

tra, and s � �2 1JCH�ÿ1

� 3:45 ls was used. The13C-NMR spectra in DMSO-d6 were also recorded

using an inverse-gated sequence. The 2D 1H±1H

phase-sensitive double-quantum ®ltered COSY

DQCOSY) spectra were typically performed with

a spectral width �600 Hz, relaxation delay 2 s,

number of increments 256 or 512 and FT size

2K � 1K. The 2D 1H±13C heteronuclear multiple

quantum coherence HMQC) and 1H±13C hetero-

nuclear multiple bond connectivity HMBC) ex-

periments were carried out with a spectral width of

�2000 Hz for 1H and 7000 Hz for 13C, relaxation

delay 1.5 s, FT size 1K � 512W for the HMQC

and 2K � 256W for HMBC.13C cross polarization magic angle spinning

total suppression of side bands CPMAS TOSS)

and 13C cross polarization magic angle spinning

non-quaternary suppression total suppression of

side bands CPMAS NQS TOSS) NMR spectra

were obtained on a Bruker MSL 300 spectrometer

operating at 75.5 MHz for 13C. 13C CPMAS TOSS

and 13C CPMAS NQS TOSS experiments were

performed at magic angle spinning MAS) rates 5

kHz, using cylindrical zircon rotor with Kel-F cap,

a contact time 2 ms and repetition rate 5 s.

Commercially available Fluka) 1-nitroso-2-

naphthol and 2-nitroso-1-naphthol were used.

3. Results and discussion

1-Nitroso-2-naphthol 1) and 2-nitroso-1-naph-

thol 2) may exist in two tautomeric forms, oxime

A and nitroso B. The possible tautomeric, isomeric

and rotameric structures of these compounds are

presented in Figs. 1 and 2. The structural forms of

A are designated as A1 for the syn-isomer, A3 for

the anti-isomer, and A2 and A4 for their rotamers,

Fig. 2. Oxime A and nitroso B tautomeric forms of 2-nitroso-1-naphthol 2.

G. Ivanova, V. Enchev / Chemical Physics 264 2001) 235±244 237

respectively. The rotamers for the nitroso tau-

tomer B are designated as B1±B4. We performed

ab initio calculations for the possible molecular

structures of compounds 1 and 2. Geometry op-

timizations C1 symmetry) were carried out with a

3-21G basis set for all species shown in Figs. 1 and

2. All attempts to locate structure A4 for 1 failed

because of steric hindrance. Then single-point 6-

31G**//3-21G calculations were performed. It

was found that B2 and B4 for 1, and A4, B2 and B4

for 2 possess too high an energy. For that rea-

son 6-31G** geometry optimizations were carried

out only for the forms A1, A2, A3, B1 and B3.

Additional single-point MP4 SDTQ)/6-31G**//6-

31G** calculations were performed for these spe-

cies to further correct for electron correlation. The

calculated total energies for the di�erent isomers

and rotamers of oxime A and the rotamers of

nitroso B tautomer in gas phase are given in Table

1. The di�erences in the energies for the species

studied are listed in Table 2. The calculations in

gas phase at HF/3-21G and HF/6-31G** levels

predict as most stable the syn-oxime form A1 for

the compounds shown in Figs. 1 and 2. However,

at HF/3-21G level the energy di�erence between

the syn- and anti-oxime isomer of 2-nitroso-1-

naphthol is 0.01 kcal/mol Table 2), i.e. these iso-

mers are isoenergetic. Improving the basis set with

polarization functions on all atoms 6-31G**//3-

21G and 6-31G**//6-31G**) leads to an increase

of this di�erence and the syn-oxime isomer A1

becomes more stable. There are small decreases in

the energy di�erences between the tautomers, iso-

mers and rotamers of the compounds investigated

in the gas phase when the scaled ZPE corrections

are taken into account. However, it can be seen

from Table 2 that the in¯uence of the correlation

energy on the relative stabilities of tautomeric

forms A1 and B1 is substantial and in the opposite

direction. The ab initio calculations at MP4-

SDTQ)/6-31G**//6-31G**+ZPE level predict that

in the gas phase the syn-oxime tautomers of

Table 1

Ab initio total energies ET, ZPE and total free energy G in solvent in hartrees) for the tautomers, isomers and rotamers of 1-nitroso-2-

naphthol and 2-nitroso-1-naphthol shown in Figs. 1 and 2

Species Gas phase PCM/6-31G**//6-31G**

6-31G**//3-21G 6-31G**//6-31G** MP4 SDTQ)/6-

31G**//6-31G**

CHCl3 DMSO

ET ET ZPE ET G G

1-Nitroso-2-naphthol

A1 ÿ586.875746 ÿ586.881594 0.161607 ÿ588.740638 ÿ586.862450 ÿ586.857367

A2 ÿ586.866544 ÿ586.872265 0.160784 ÿ588.730527 ÿ586.855772 ÿ586.851392

A3 ÿ586.867912 ÿ586.873342

B1 ÿ586.871376 ÿ586.876776 0.160813 ÿ588.733276 ÿ586.855505 ÿ586.849586

B2 ÿ586.855204

B3 ÿ586.868759 ÿ586.873353

B4 ÿ586.855484

TS A1±A2) ÿ586.862761 0.159354 ÿ588.719417 ÿ586.845799 ÿ586.841608

TS A1±B1) ÿ586.862157 0.156207 ÿ588.728325 ÿ586.842989 ÿ586.837887


A1 ÿ586.878664 ÿ586.884498 0.161706 ÿ588.743314 ÿ586.864832 ÿ586.859542

A2 ÿ586.870728 ÿ586.876451 0.161183 ÿ588.734335 ÿ586.858776 ÿ586.854348

A3 ÿ586.877246 ÿ586.882563 0.161326 ÿ588.739160 ÿ586.865531 ÿ586.861413

A4 ÿ586.862240

B1 ÿ586.873112 ÿ586.878628 0.160902 ÿ588.734766 ÿ586.856859 ÿ586.850739

B2 ÿ586.853938

B3 ÿ586.873940 ÿ586.878452 0.160927 ÿ588.733234 ÿ586.857021 ÿ586.851062

B4 ÿ586.859645

TS A1±A2) ÿ586.865477 0.159558 ÿ588.721638 ÿ586.847998 ÿ586.843641

TS A1±B1) ÿ586.863860 0.156380 ÿ588.729833 ÿ586.843861 ÿ586.838390


compounds 1 and 2 are more stable than the

nitroso ones by 4.17 and 4.91 kcal/mol, respec-

tively.

The structures of 1-nitroso-2-naphthol and 2-

nitroso-1-naphthol in solid state and in solution

were established on the basis of NMR spectral

investigations. The complete assignments of the 1H

NMR and 13C NMR spectra of 1 and 2 in CDCl3,

DMSO-d6 and DMSO-d6 � acetic acid have been

achieved on the basis of DQ ®ltered COSY, 1H/13C

HMQC and 1H/13C HMBC experiments.

3.1. 1-Nitroso-2-naphthol

The assignments of the proton spectra of 1 in

the three solvents are in agreement with the liter-

ature data except for protons H5, H6 and H7,

which are reverse [1±5] and the chemical shifts are

now in the right order Table 3). The carbon NMR

data for compound 1 in CDCl3, DMSO-d6 and

DMSO-d6 � CD3COOD solutions at 300 K are

presented in Table 4. The 13C NMR data are in

agreement with published data [4,19] and con®rm

existence of tautomer A only. However, we ob-

serve a doubling of the resonance peaks for all CH

carbons in the carbon spectrum in chloroform

solution but the presence of only one form in

DMSO solution. We assume the existence of two

rotamers of the oxime form of 1 in chloroform.

According to ab initio PCM calculations the en-

ergy di�erence between rotamers A1 and A2 as well

as the rotational barrier between them decrease

with the increasing of the solvent polarity from

chloroform to dimethylsulfoxide Fig. 3). This dif-

ference in the rotational barrier heights could ex-

plain the presence of the resonance peaks of the

rotameric forms, A1 and A2 observed by us in the

carbon NMR spectrum in chloroform solution.

The ab initio calculated bond lengths of syn-1,2-

naphthoquinone-1-oxime A1 given in Table 5 are

in agreement with available X-ray data [6]. A

comparison of the calculated and experimental

data shows that the di�erences in the C±C bond

lengths do not exceed 0.018 �A. There are more

substantial changes in the C±O, C±N and N±O

bond lengths, which are shorter by 0.041, 0.044

and 0.042 �A, respectively. The experimental and

calculated data show that an intramolecular hy-

drogen bond is formed Table 5). The calculations

predict a longer by 0.073 �A) interatomic distance

between the oxygen atoms O12 and O13 partici-

pating in the intramolecular hydrogen bond.

Table 2

Calculated relative stabilities in kcal/mol) for all possible species of 1-nitroso-2-naphthol and 2-nitroso-1-naphthol, shown in Figs. 1

and 2, at di�erent computational levels in the gas phase and in solution CHCl3 and DMSO)

Computational level A1 A2 A3 A4 B1 B2 B3 B4


3-21G//3-21G 0.00 6.22 4.19 5.43 19.84 8.62 18.53

6-31G**//3-21G 0.00 5.77 4.92 2.74 12.89 4.38 12.71

6-31G**//6-31G** 0.00 5.88 5.18 3.02 5.17

6-31G**//6-31G**+ZPE 0.00 5.39 2.58

MP4 SDTQ)/6-31G**//6-31G**+ZPE 0.00 5.88 4.17

PCM/6-31G**//6-31G**+ZPEa 0.00 3.73 3.91

PCM/6-31G**//6-31G**+ZPEb 0.00 3.29 4.44


3-21G//3-21G 0.00 5.15 0.01 13.10 6.17 23.55 8.29 19.26

6-31G**//3-21G 0.00 4.98 0.89 10.30 3.48 15.52 2.96 11.93

6-31G**//6-31G** 0.00 5.05 1.21 3.68 3.79

6-31G**//6-31G**+ZPE 0.00 4.76 1.00 3.23 3.36

MP4 SDTQ)/6-31G**//6-31G**+ZPE 0.00 5.34 2.39 4.91 5.89

PCM/6-31G**//6-31G**+ZPEa 0.67 4.16 0.00 5.20 5.12

PCM/6-31G**//6-31G**+ZPEb 1.41 4.35 0.00 6.46 6.27

a In CHCl3.b In DMSO.


3.2. 2-Nitroso-1-naphthol

As can be seen from Table 2 the energy di�er-

ence in the gas phase between isomers A3 and A1

for 1,2-naphthoquinone-2-oxime at 6-31G**//6-

31G** level is calculated to be 1.21 kcal/mol.

When corrected for ZPE using the 6-31G** vib-

rational frequencies the di�erence between these

isomers comes down to 1.00 kcal/mol. If in addi-

tion the correlation energy is taken into account

the di�erence increases to 2.39 kcal/mol. However,

the ab initio PCM results at HF/6-31G** level

shown in Fig. 4 indicate that when the sol-

vent e�ect is accounted for the A3 isomer of 1,2-

Table 31H NMR chemical shifts ppm, TMS) and coupling constants J Hz) in italics) of compounds 1 and 2 in CDCl3, DMSO-d6 and

DMSO-d6 � acetic acid

H Compound 1 Compound 2

CDCl3 DMSO DMSO � acid CDCl3 DMSO DMSO � acid

OH 17.45 s 14.28 s 17.02 br.s. 13.63 br.s.

H3 6.53 d, 9.8 6.37 d, 9.7 6.32 d, 9.8 6.89 d, 9.9 7.10 d, 10.1 7.06 d, 10.0

6.94a)

H4 7.66 d, 9.8 7.65 d, 9.7 7.61 d, 9.8 6.96 d, 9.9 7.00 d, 10.1 6.90 dd,

7.20 d, 9.8) 10.0; 1.4

H5 7.45a 7.5±7.4a 7.5±7.4a 7.46 d, 7.3 7.51 d, 7.7 7.40a

7.38 d, 7.3)

H6 7.5a 7.5±7.4a 7.5±7.4a 7.72 td, 7.6; 1.2 7.67 ddd, 7.58 ddd,

7.63 td, 7.6; 1.2) 7.7; 7.4; 1.3 7.5; 6.0; 1.5

H7 7.46a 7.5±7.4a 7.5±7.4a 7.49 td, 9.0; 1.4 7.48 ddd, 7.40a

7.42 t, 8.2; 1.2) 7.7; 7.4; 1.1

H8 8.26 dd, 8.85 br.s. 8.81 br.s. 8.35 dd, 7.9; 1.2 8.10 d, 7.7 7.97 d, 8.0

6.3; 2.0 8.20 d, 7.6)

The 1H NMR data for the possible rotamers or isomers are presented in brackets.a Signal overlap.

Table 413C NMR chemical shifts ppm, TMS) of compounds 1 and 2 in CDCl3, DMSO-d6 and DMSO-d6 � acetic acid and solid state

C Compound 1 Compound 2

CDCl3 DMSO DMSO � acid Solid state CDCl3 DMSO DMSO � acid Solid state

C1 144.8 145.1 142.2 145.8 182.2 181.3 182.2 183.7

180.9)

C2 182.8 183.3 182.2 183.8 147.1 147.8 148.4 146.8

C3 125.7 127.5 126.6a 126.9 125.3 116.0 116.8 113.6

127.1) 115.7)

C4 147.9 145.1 144.4 148.8 127.3 130.3 131.1 131±136a

146.2)

C5 129.7 129±131a 129.5a 130±127a 128.4 129.0 129.7 126.5a

130.3) 129.0)

C6 131.0 129±131a 129.9a 130±127a 136.1 134.7 135.4 131±136a

131.4) 134.9)

C7 129.7 129±131a 129.5a 130±127a 128.2 129.2 129.9 126.5a

130.3) 129.2)

C8 123.0 129±131a 126.6a 125.2 128.7 127.0 127.9 126.5a

126.2) 127.9)

C9 130.6 129±131a 129.9a 130±127a 137.0 131.2 132.1 130.3

C10 128.5 126.8 129.5a 130±127a 137.6 136.4 137.3 138.2

The chemical shifts for the possible rotamers or isomers are presented in brackets. For numbering of the atoms see Figs. 1 and 2.a Signal overlap.


naphthoquinone-2-oxime becomes most stable in

energy.

The proton NMR data for 2-nitroso-1-naph-

thol in all solvents used are presented in Table 3.

We observe di�erences from the published data

in two groups of protons: H5/H6/H7 in all solu-

tions) [4] and H3/H4 in DMSO-d6 and DMSO-

d6 � acetic acid) [3,4], where the assignments are

reversed. The correct assignment of the proton

chemical shifts is proved on the basis of the data

from 1H/13C HMQC and 1H/13C HMBC experi-

ments. Our analysis of the 1H NMR spectra of 2

indicates the presence of one form in DMSO-d6

and DMSO-d6 � acetic acid, but in CDCl3 we

observe detectable amounts of two species of 2. As

can be seen in Fig. 4, the solvent e�ect is more

pronounced in a polar medium DMSO). The

calculated energy di�erence between A3 and A1

increases to 1.41 kcal/mol which could explain the

presence of the A3 isomer only. The ab initio cal-

culated di�erence between the relative stabilities of

the A3 and A1 isomers is found to be only 0.67

kcal/mol in CHCl3, and hence the coexistence of

both isomers. The carbon NMR data show the

existence of the oxime tautomer of 2 only, in all

solutions. Our structural elucidation of 2-nitroso-

1-naphthol in CDCl3 is in disagreement with the

latest published data [4] where both tautomeric

forms are claimed to be presented. The detailed

analysis of the 13C NMR spectrum in CDCl3con®rms the oxime structure of both species. The

structures of the isomeric forms A1 and A3 of 2 are

con®rmed by the 13C NMR data. The resonance

peaks at 182.2 and 180.9 ppm are assigned to C1 of

Fig. 3. Ab initio PCM/6-31G**//6-31G**+ZPE) calculated relative stabilities in kcal/mol) of the tautomers and rotamer of 1-nitroso-

2-naphthol and barriers of tautomerization and rotation in CHCl3 normal) and DMSO italics). The values of the ZPEs used for the

solvated species are those calculated for the unsolvated ones.


isomers A1 and A3, respectively. The calculations

predict an interatomic distance of 2.6028 �A Table

5) between the oxygen atoms O12 and O13, i.e.

there is an intramolecular hydrogen bond. The

existence of an intramolecular hydrogen bond in

the syn-isomer A1 leads to a down®eld chemical

shift of C1 by 1.3 ppm. We also observe changes in

the chemical shift of C3 b-carbon chemical shift)

associated with the cis±trans position of the oxime

group at C2 [20,21]. A low®eld shift of the reso-

nance peak of C3 of the syn-isomer A1 but up®eld

shift of C3 of the anti-isomer A3 are detected

Table 4). The resonance at 125.3 ppm is assigned

to C3 of the syn-isomer A1 but that at 115.7 ppm

to C3 of the anti-isomer A3.

3.3. Solid state 13C NMR data

The solid state 13C NMR data indicate the pres-

ence of compounds 1 and 2 in their oxime forms

only Table 4). The resonance assignment of the13C CPMAS NMR spectrum of 2 and more spe-

Table 5

Ab initio 6-31G** calculated interatomic distances in �A) for the oxime forms A1 of compound 1 and A1 and A3 of compound 2 in gas

phase

Distances Compound 1 ± syn-oxime A1

C1±C2 1.4999 1.485)

C2±C3 1.4623 1.444)

C3±C4 1.3310 1.328)

C4±C10 1.4622 1.451)

C5±C10 1.3935 1.397)

C9±C10 1.3970 1.411)

C5±C6 1.3784 1.377)

C6±C7 1.3890 1.394)

C7±C8 1.3810 1.385)

C8±C9 1.3916 1.390)

C1±C9 1.4862 1.477)

C2±O13 1.2089 1.250)

C1±N11 1.2705 1.315)

N11±O12 1.3216 1.364)

O12±H 0.9570 0.726)

O13±H 1.7474 1.886)

O12±O13 2.5826 2.510)

Compound 2

syn-oxime A1 anti-oxime A3

C1±C2 1.4966 1.5131

C2±C3 1.4652 1.4684

C3±C4 1.3260 1.3265

C4±C10 1.4668 1.4687

C5±C10 1.3923 1.3900

C9±C10 1.4003 1.3980

C5±C6 1.3814 1.3837

C6±C7 1.3899 1.3868

C7±C8 1.3791 1.3830

C8±C9 1.3921 1.3878

C1±C9 1.4774 1.4911

C1±O11 1.2084 1.1924

C2±N12 1.2730 1.2617

N12±O13 1.3215 1.3534

O13±H 0.9562 0.9435

O11±H 1.7711 4.4058

O11±O13 2.6028 4.0333


ci®cally the chemical shift of C3 indicates the exis-

tence of the anti-1,2-naphthoquinone-2-oxime A3.

4. Concluding remarks

On the basis of NMR spectroscopy and ab initio

quantum-chemical calculations, only the oxime

tautomeric form of 1-nitroso-2-naphthol and 2-

nitroso-1-naphthol is found to exist. The syn-1,2-

naphthoquinone-1-oxime A1 is the most stable one

in the gas phase, solution and solid state. Two

rotameric forms of this tautomer are observed in

solution by us for the ®rst time. It can be seen from

the Table 6 that the barrier height of rotation

around the C±O bond A1 ! A2 process) in the gas

phase increases when the electron correlation at

MP4 level) is taken into account but the increase

is not substantial. However, there is a decrease

of the barrier height of rotation in solution, in

comparison with the gas phase see Table 6 and

Figs. 3 and 4). Increasing the solvent polarity leads

to a lowering of the rotational barrier Figs. 3

and 4).

Fig. 4. Ab initio PCM/6-31G**//6-31G**+ZPE) calculated relative stabilities in kcal/mol) of the tautomers, isomer and rotamer of 2-

nitroso-1-naphthol and barriers of tautomerization and rotation in CHCl3 normal) and DMSO italics). The values of the ZPEs used

for the solvated species are those calculated for the unsolvated ones.

Table 6

Calculated barriers in kcal/mol) of tautomerization A1 $ B1 and of rotation A1 $ A2 in the gas phase

Computational level A1 ! B1 B1 ! A1 m# A1 ! A2 A2 ! A1 m

#


6-31G**//6-31G** 12.20 9.18 1625i 11.82 5.97 370i

6-31G**//6-31G**+ZPE 9.17 6.59 10.56 5.17

MP4 SDTQ)/6-31G**//6-31G**+ZPE 4.70 0.53 12.05 6.17


6-31G**//6-31G** 12.95 9.27 1636i 11.94 6.89 402i

6-31G**//6-31G**+ZPE 9.96 6.23 10.73 5.97

MP4 SDTQ)/6-31G**//6-31G**+ZPE 5.48 0.57 12.40 7.06

Imaginary frequencies m# are scaled by 0.893.


The calculations at a 6-31G**//6-31G** and

MP4 SDTQ)/6-31G**//6-31G**+ZPE level pre-

dict 1,2-naphthoquinone-2-oxime to exists as syn-

oxime isomer A1 in the gas phase. However, our

experimental and theoretical results prove the

presence of syn- and anti-oxime isomers, A1 and

A3, in solution. We have observed a similar co-

existence of syn- and anti-isomers of acenaph-

thenedionemonoxime [20]. In the solid state 13C

CPMAS NMR spectrum, resonance peaks of the

anti-oxime isomer A3 are observed.

According to the literature data [3,4], both

tautomeric forms of 2-nitroso-1-naphthol are

present in solution. However, in the 13C NMR

spectra of 2 in solution, the characteristic reso-

nance signal for an aromatic carbon with attached

hydroxy group in the range 155±170 ppm [20,22±

24] has not been observed. Tautomers A1 and

B1 are interconvertible by intramolecular proton

transfer. It can be seen from Table 6 that the in-

clusion of electron correlation in the calculations

strongly in¯uences the tautomerization barrier.

The barriers of tautomerization in the gas phase

for compounds 1 and 2 at the MP4 SDTQ)/6-

31G**//6-31G**+ZPE level are predicted to be

4.70 and 5.48 kcal/mol, respectively. In the gas

phase, the barrier of tautomerization is found to

be lower than that of rotation Table 6) and maybe

in this case tautomerization could occur. In solu-

tion, however, the barrier of tautomerization cal-

culated at HF/6-31G** level is found to be higher

than the rotation one Figs. 3 and 4) and this could

explain the absence of tautomer B1. It can be seen

from Figs. 3 and 4 that solvent polarity has almost

no in¯uence on the barrier height of tautomeriza-

tion between A1 and B1.

Acknowledgements

We thank Dr. A.E. Aliev, University College,

London, UK, for recording the solid state 13C

NMR spectra.

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Does tautomeric equilibrium exist in ortho-nitrosonaphthols?

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