water-assisted proton transfer in formamide, thioformamide and selenoformamide

Water-assisted proton transfer in formamide, thioformamide

and selenoformamide

Nadezhda Markova, Venelin Enchev*

Institute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Received 20 February 2004; revised 5 April 2004; accepted 8 April 2004

Abstract

The tautomeric equilibria of formamide, thioformamide and selenoformamide were studied in the gas phase and in water solution using ab

initio quantum chemical calculations. The solvent effects were considered by explicit inclusion of three water molecules, which model a first

hydration shell around the solute. Full geometry optimizations of these complexes were carried out at MP2/6-31G(d,p) and MP2/6-31 þ G(d)

levels of theory. Single point calculations were performed at MP4/6-31 þ G(d)//MP2/6-31 þ G(d) computational level to obtain more

accurate energies. The basis set and electron correlation effects on the energy barriers of tautomerization and the energy differences between

tautomers were analyzed. The minimum energy paths for water-assisted proton transfer in tri-hydrated formamide and its chalcogen

analogues thioformamide and selenoformamide were followed at MP2/6-31 þ G(d) level.

q 2004 Elsevier B.V. All rights reserved.

Keywords: Selenoformamide; Tautomerization; Tautomers; Formamide; Thioformamide; Assisted proton transfer

1. Introduction

Formamide and its thio and seleno analogues can exist

theoretically in two tautomeric forms—amino A and imino

B (Fig. 1). Experimental results indicate clearly that the

amino tautomers of formamide [1–4], thioformamide [5,6]

and selenoformamide [7,8] are thermodynamically more

stable than the imino tautomers in vapor phase, inert

environment and polar medium.

Much theoretical work has been devoted to the study of

the properties of formamide and its structure in aqueous

solution. Classical Monte Carlo [9 – 11], molecular

dynamics [12–14] simulations and quantum chemical

calculations using the dielectric continuum solvent model

[15] have been used. Monohydrated formamide–water

complexes have been studied at ab initio and DFT level

by several researches [16–21]. Since the proton transfer

occurs in aqueous solution, water can act not only as a

solvent but also as a mediator which gives and accepts

protons to promote proton transfer. The barrier for the

prototropic tautomerism in this system is reduced by more

than 25 kcal mol21 when a water molecule mediates

the proton transfer [22–26]. Simons et al. [24] have found

that the tunneling effect is large and lowers the barrier about

4.6 kcal mol21.

The theoretical results by Leszczynski et al. [27] and

Dapprich and Frenking [28] obtained for thioformamide and

selenoformamide are in agreement with experiment [5–8]

that the amino tautomer A is more stable than the imino

tautomer B. Their ab initio calculations predict that the

thione form of thioformamide is 11.70 kcal mol21 lower in

energy than the thiol form and that the selenone form is

13.80 kcal mol21 more stable than the selenol form [28].

Unlike formamide, however, there is no data available on

the behavior of thioformamide and selenoformamide in

water solution.

In the present paper we consider the influence of the

intermolecular hydrogen bonds formed between the solute

and solvent molecules on: (i) the energy difference between

the amino and imino tautomers of formamide, thioforma-

mide and selenoformamide, and (ii) the energy barrier of

their tautomerization. Ab initio calculations were carried

out at MP2/6-31G(d,p) and MP2/6-31 þ G(d) compu-

tational levels. For comparison, at the same levels of theory,

calculations for the isolated molecules and direct proton

transfer were also performed.

0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2004.04.018

Journal of Molecular Structure (Theochem) 679 (2004) 195–205

www.elsevier.com/locate/theochem

* Corresponding author. Tel.: þ3592-960-6197; fax: þ3592-870-0225.

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

http://www.elsevier.com/locate/theochem

2. Computational details

Ab initio calculations using many-body perturbation

theory (MBPT) were carried out for the study of the

interaction of formamide, thioformamide and selenoforma-

mide tautomers with one and three water molecules. The

geometries of the minima and the transition structures for the

tautomeric conversions were located using standard

6-31G(d,p) and 6-31 þ G(d) basis sets without symmetry

constraints (C1 symmetry was assumed) by the gradient

procedure at second order closed shell restricted Møller–

Plesset perturbation level of theory. Local minima and

transition states (TS) were verified by establishing that the

matrices of the energy second derivatives (Hessians) have

zero and one negative eigenvalues, respectively. To obtain

accurate energies, single-point calculations with extended

basis sets up to MP4/6-311þþG(3df,2p)//MP2/6-31 þ G(d)

level of theory were also performed. The total energies were

corrected using MP2/6-31G(d,p) and MP2/6-31 þ G(d) zero

point energies scaled by a factor of 0.945. The scaled ZPE

corrections are included in the relative energy values.

To establish the connection between the transition

structure and corresponding equilibrium structures in the

tri-hydrated complex of formamide and its chalcogen

analogues, the reaction pathway was followed using the

intrinsic reaction coordinate (IRC) procedure. The fourth-

order Runge–Kutta (RK4) algorithm [29,30] implemented

in the GAMESS program package was used to obtain the

IRC of the water-mediated PT reaction. The reaction path

was followed from the transition state to the reactant and to

the product using a step size of 0.05 au1/2 bohr.

The calculations were carried out using the PC GAMESS

version [31] of the GAMESS (US) quantum chemistry

package [32].

3. Results and discussion

Formamide, thioformamide and selenoformamide can

give two tautomeric forms as a result of the amino–imino

equilibria: the amino and imino tautomers shown in Fig. 1.

Geometry optimizations were carried out at MP2 level with

two different basis sets for both tautomers and their hydrated

complexes with one and three water molecules. The

calculated total energies and energy differences are given

in Tables 1 and 2, respectively.

3.1. Formamide

A series of single-point energy calculations using the

MP2/6-31G(d,p) and MP2/6-31 þ G(d) geometries were

performed for all structures investigated at MP2 and MP4

levels with the extended basis sets 6-311G(d,p) and

6-311þþG(3df,2p), to investigate the effects of basis set

and electron correlation. To obtain the true energy a

zero-point energy correction was added to the total

energy. Formamide is calculated to be more stable than

formamidinic acid by 12.81 and 14.43 kcal mol21 at

MP2/6-31G(d,p) and MP2/6-31 þ G(d) level, respectively.

Basis set improvements at the MP2 and MP4 level result in

decreases in the energy difference (see Fig. 2). The same

Fig. 1. The tautomeric forms of formamide 1, thioformamide 2 and

selenoformamide 3.

Table 1

Total energies (ET) and zero-point energy (ZPE) corrections (a.u.)

calculated at MP2/6-31G(d,p) and MP2/6-31 þ G(d) levels for formamide,

thioformamide and selenoformamide

Species MP2/6-31G(d,p) MP2/6-31 þ G(d)

ET ZPE ET ZPE

Formamide

A 2169.421145 0.046277 2169.410435 0.045760

B 2169.401478 0.047064 2169.387934 0.046281

TS 2169.346585 0.041526 2169.331663 0.040556

A 1 H2O 2245.661368 0.073137 2245.637584 0.071535

B 1 H2O 2245.644710 0.073886 2245.618188 0.072251

TS 1 H2O 2245.625570 0.067760 2245.595304 0.065770

A 1 3H2O 2398.126728 0.123260 2398.081678 0.119635

B 1 3H2O 2398.104235 0.122628 2398.056326 0.119405

TS 1 3H2O 2398.088049 0.117500 2398.036831 0.113362

Thioformamide

A 2492.002911 0.044741 2491.985817 0.044091

B 2491.982385 0.041496 2491.964412 0.041019

TS 2491.935096 0.038332 2491.915039 0.037511

A 1 H2O 2568.241339 0.071276 2568.211922 0.069605

B 1 H2O 2568.219330 0.067717 2568.189740 0.066149

TS 1 H2O 2568.203171 0.064382 2568.169652 0.062922

A 1 3H2O 2720.711106 0.121997 2720.654518 0.117250

B 1 3H2O 2720.696438 0.119022 2720.633739 0.115289

TS 1 3H2O 2720.672887 0.115702 2720.617812 0.113328

Selenoformamide

A 22492.080471 0.044234 22492.085224 0.043755

B 22492.061044 0.039696 22492.061478 0.039336

TS 22492.017181 0.037269 22492.017390 0.036704

A 1 H2O 22568.322467 0.070696 22568.313950 0.069162

B 1 H2O 22568.297418 0.065828 22568.287132 0.064476

TS 1 H2O 22568.284832 0.063419 22568.272090 0.062341

A 1 3H2O 22720.797788 0.121279 22720.769652 0.119266

B 1 3H2O 22720.770296 0.116742 22720.737595 0.114258

TS 1 3H2O 22720.761753 0.114948 22720.725925 0.113000

N. Markova, V. Enchev / Journal of Molecular Structure (Theochem) 679 (2004) 195–205196

Table 2

Calculated energy differences DH0 and energy barriers DH # in kcal mol21 for formamide, thioformamide and selenoformamide, and their complexes with one

and three water molecules. The imaginary frequencies n # are given in cm21

Computational level DH0 DH # n # DH0 DH # n # DH0 DH # n #

Formamide Formamide þ H2O Formamide þ 3H2O

MP2/631G(d,p)//MP2/6-31G(d,p) 12.81 43.97 1822I 10.90 19.27 1562i 13.74 20.86 1526i

MP4/631G(d,p)//MP2/6-31G(d,p) 12.13 47.12 10.73 22.76 13.65 24.31

MP2/631 þ G(d)//MP2/6-31 þ G(d) 14.43 46.34 1844i 12.60 23.11 1660i 15.76 24.21 1610i

MP4/6-31 þ G(d)//MP2/6-31 þ G(d) 13.59 49.47 12.23 26.89 15.35 27.89

Thioformamide Thioformamide þ H2O Thioformamide þ 3H2O

MP2/6-31G(d,p)//MP2/6-31G(d,p) 10.96 38.75 1624i 11.70 19.86 1218i 7.44 20.25 953i

MP4/6-31G(d,p)//MP2/6-31G(d,p) 9.29 41.54 10.43 23.12 6.42 23.42

MP2/6-31 þ G(d)//MP2/6-31 þ G(d) 11.61 40.51 1665i 11.87 22.56 1325i 11.88 20.71 963i

MP4/6-31 þ G(d)//MP2/6-31 þ G(d) 10.00 43.38 10.66 26.39 10.86 24.50

Selenoformamide Selenoformamide þ H2O Selenoformamide þ 3H2O

MP2/6-31G(d,p)//MP2/6-31G(d,p) 9.50 35.58 1506i 12.83 19.30 1009i 14.56 18.86 845i

MP4/6-31G(d,p)//MP2/6-31G(d,p) 7.62 37.95 11.36 22.09 13.16 21.49

MP2/6-31 þ G(d)//MP2/6-31 þ G(d) 12.28 38.39 1553i 14.05 22.22 1097i 17.15 23.72 878i

MP4/6-31 þ G(d)//MP2/6-31 þ G(d) 10.59 41.06 12.66 25.48 15.69 26.83

DH0 ¼ DETðA 2 BÞ þ 0:945DZPEðA 2 BÞ;DH# ¼ DETðA 2 TSÞ þ 0:945DZPEðA 2 TSÞ:

Fig. 2. Variation of the calculated energy differences (DH0) and energy barrier heights (DH #) for formamide and its hydrated complexes with computational

level of theory used. DH0 and DH # are in kcal mol21.

N. Markova, V. Enchev / Journal of Molecular Structure (Theochem) 679 (2004) 195–205 197

tendency is observed for the complexes of formamide with

one and three water molecules. However, the energy

difference for the monohydrated complexes is lower than

that for the isolated tautomers while for the tri-hydrated

complexes it is higher. At MP4/6-311þþG(3df,2p)//MP2/

6-31 þ G(d) þ ZPE level of theory, the calculated tauto-

merization energies are 11.73, 10.75 and 13.97 kcal mol21

for the isolated tautomers, monohydrated and tri-hydrated

complexes, respectively. The influence of the computational

level (basis set and electron correlation) on the energy

differences DH0 between tautomers A and B, as well as

between their water complexes, is shown in Fig. 2.

The ab initio optimized geometries of tautomers A and B

and their complexes with water are collected in Table 3. The

available experimental structural data are also included.

The calculated bond lengths in formamide at MP2/

6-31 þ G(d,p) level are very near to the experimental

values taken from microwave spectroscopy. Our MP2

calculations predict a nonplanar structure of formamide.

The amino group is turned 5.58 towards the formyl moiety

as shown by calculations at MP2/6-31 þ G(d) level. It is

known that the results are sensitive to basis sets effects and

electron correlation [33]. While HF calculations may give

both planar and nonplanar geometries depending on the

basis set, inclusion of electron correlation in the framework

of second order perturbation theory leads to preference of

nonplanarity. An exactly planar structure of formamide has

been found at CCSD(T)/cc-PVTZ level by Fogarasi and

Szalay [33].

The formation of an adduct with one water molecule

induces some changes in the geometrical parameters of

formamide. According to calculations at MP2/6-31 þ G(d)

level the N1–C2 and C2–H6 bonds are shortened by 0.0113

and 0.0020 A, respectively, while the carbonyl C2yO3 bond

is lengthened by 0.0104 A. The formation of an inter-

molecular hydrogen bond in the A 1 H2O complex leads to

a lengthening of the N1–H4 bond by 0.0072 A while the

N1–H5 bond almost does not change. The effect of

complexation is substantial on the water molecule. There

is a lengthening by 0.0128 A of the O–H bond participating

in the intermolecular hydrogen bond (Fig. 3). The

calculations predict the O8· · ·H4 distance to be longer

Table 3

Selected MP2/6-31 þ G(d) interatomic distances (A) for formamide, thioformamide and selenoformamide, and their tri-hydrated complexes and transition

states. Available experimental data are also given in brackets. For numbering of the atoms, see Figs. 3–5

Distance Formamide Thioformamide Selenoformamide

A A 1 3H2O A A 1 3H2O A A 1 3H2O

N1–C2 1.3630 (1.352)a 1.3386 1.3515 (1.358)b 1.3307 1.3470 1.3242

C2yX3 1.2292 (1.219)a 1.2517 1.6358 (1.626)b 1.6630 1.7680 1.8009

C2–H6 1.1031 (1.098)a 1.0990 1.0931 (1.096)b 1.0920 1.0919 1.0905

N1–H4 1.0133 (1.002)a 1.0200 1.0143 (1.006)b 1.0236 1.0150 1.0259

N1–H5 1.0107 (1.002)a 1.0169 1.0120 (1.001)b 1.0195 1.0127 1.0197

O8–H4 2.0692 1.9679 1.9296

O8–H7 0.9839 0.9819 0.9831

O8–H9 0.9709 0.9719 0.9720

X3–H7 1.9333 2.4574 2.5374

B B 1 3H2O B B 1 3H2O B B 1 3H2O

N1yC2 1.2770 1.2815 1.2848 1.2847 1.2839 1.2844

C2–X3 1.3531 1.3538 1.7521 1.7600 1.8857 1.8931

C2–H6 1.0909 1.0905 1.0954 1.0933 1.0964 1.0925

X3–H7 0.9815 0.9977 1.3430 1.3489 1.4768 1.4825

N1–H5 1.0212 1.0228 1.0253 1.0255 1.0265 1.0262

N1–H4 1.9628 2.0142 2.0124

O8–H4 0.9892 0.9828 0.9829

O8–H9 0.9717 0.9739 0.9743

O8–H7 1.7997 2.2395 2.2091

TS TS 1 3H2O TS TS 1 3H2O TS TS 1 3H2O

N1–C2 1.3080 1.3026 1.3121 1.3005 1.3099 1.2961

C2–X3 1.2943 1.3122 1.6960 1.7381 1.8321 1.8799

C2–H6 1.0886 1.0917 1.0894 1.0931 1.0903 1.0923

N1–H5 1.0179 1.0206 1.0251 1.0222 1.0270 1.0235

N1–H4 1.3523 1.2989 1.4009 1.5804 1.4249 1.6414

X3–H7 1.3512 1.1766 1.6958 1.6841 1.7991 1.7756

O8–H7 1.2921 1.2080 1.2367

O8–H4 1.2216 1.0605 1.0425

O8–H9 0.9741 0.9853 0.9866

X ¼ O, S, Se (see Fig. 1).a Ref. [1].b Ref. [6].


than the O3· · ·H7 one by 0.0908 A. The intermolecular

distances O8· · ·H4 (1.99 A) and O3· · ·H7 (2.03 A) in the

monohydrated complex have been determined experimen-

tally by Lovas et al. [34]. The experimental data show the

opposite: O8· · ·H4 distance is shorter by 0.04 A compared

to the O3· · ·H7 distance. This discrepancy could be due to

the fact that the experimental structure is obtained from a

model which neglects the intra- and intermolecular

vibrational averaging of the measured rotation constants

as was pointed out by Fu et al. [22].

For the complex A 1 3H2O the shortening of the

N1–C2 and C2–H6 bonds and the lengthening of the

C2yO3 bond are more strongly expressed. The N1–H4 and

N1–H5 bonds are also lengthened because of the inter-

molecular hydrogen bonding (Fig. 3 and Table 3).

Calculations at MP2/6-31 þ G(d) level of theory predict

shorter H· · ·O distances (i.e. stronger intermolecular hydro-

gen bonds) than those at MP2/6-31G(d,p) level. The

stationary-point geometries of the complexes of

A 1 3H2O and B 1 3H2O, calculated at MP2/

6-31 þ G(d) level, are shown in Fig. 3.

The geometry changes mentioned above are reflected

also in the calculated IR spectra of A, A 1 H2O and

A 1 3H2O listed in Table 4. The lengthening of the

carbonyl bond corresponds to a shift of the characteristic

frequency to low frequencies. The intensity of the band is

found to be lower. The same tendency is observed for the

N–H stretching vibrations. It can be seen from Table 4 that

the MP2/6-31 þ G(d) calculated frequencies, scaled by a

factor of 0.945, are lower than the experimental ones. An

exception is yCH (þ62 cm21).

The transition state structures corresponding to the direct

and assisted proton transfer reactions were computed. The

predicted TS were verified by establishing that the Hessians

have only one negative eigenvalue. The calculated barriers

of the tautomerization reactions for isolated, monohydrated

and tri-hydrated formamide and the respective imaginary

frequencies, calculated at different levels of theory, are

presented in Table 2. The energy barriers were corrected for

the zero-point energy (ZPE) obtained at MP2/6-31G(d,p)

and MP2/6-31 þ G(d) levels of harmonic vibrational

frequency calculations and scaled by a factor of 0.945.

The transition state geometry corresponding to water-

assisted proton transfer in the tri-hydrated complex is

shown in Fig. 3.

Table 2 shows that direct proton transfer is characterized

by a high activation energy. Inclusion of a water molecule

(monohydrated complex) drastically reduces the activation

energy. For the tri-hydrated complex the proton transfer

activation energy is higher than that for the monohydrated

complex by 1–1.5 kcal mol21 at different computational

levels. At the highest level of calculation, MP4/

6-311þþG(3df,2p)//MP2/6-31 þ G(d) þ ZPE, DH # is

predicted to be 23.54 and 24.89 kcal mol21 for mono- and

tri-hydrated complexes, respectively. The influence of

Fig. 3. Formamide–water complexes. The geometries were optimized at

MP2/6-31 þ G(d) level of theory. Mulliken charges (e2) on the atoms are

given.


the computational level on the calculated energy barrier is

shown in Fig. 2.

3.2. Thioformamide and selenoformamide

The tautomerisation processes in the thio and seleno

analogues of formamide could also occur by direct or water-

assisted proton transfer. Both mechanisms were investigated

at MP2/6-31G(d,p) and MP2/6-31 þ G(d) levels of theory.

The calculated total energies and zero-point energies for

the amino and imino forms and transition states of 2 and 3

(Fig. 1) are listed in Table 1. Table 2 shows the energy

differences between the tautomers and the barriers of

tautomerization in gas phase and water solution. MP4/

6-31G(d,p)//MP2/6-31G(d,p) and MP4/6-31 þ G(d)//MP2/

6-31 þ G(d) single-point energy calculations were per-

formed for all structures investigated. The zero-point energy

correction was also added to the total energies.

At all levels of theory the amino forms 2A and 3A are

more stable than the respective imino forms 2B and 3B. The

energy differences between the amino and imino forms of 3

are 9.50 kcal mol21 at MP2/6-31G(d,p) level of optimiz-

ation and 12.28 kcal mol21 at MP2/6-31 þ G(d) level.

These energy differences for 2 are 10.96 kcal mol21 at

Table 4

MP2/6-31 þ G(d) calculated IR data for formamide, thioformamide and selenoformamide, and their mono- and tri-hydrated complexes. The frequencies,

scaled by 0.945, are in cm21 and intensities (in brackets) are in km mol21. Available experimental data are given for comparison

Assignment A A (exptl) A 1 H2O A 1 3H2O

Formamide

N–H anti sym str 3549 (61) 3563a 3533 (121) 3453 (522)

N–H sym str 3399 (58) 3439a 3343 (11) 3313 (160)

CH str 2916 (84) 2854a 2900 (87) 2950 (38)

CO str þ HCN bend þ CN str 1694 (438) 1754a 1685 (352) 1673 (367)

HNC bend 1558 (53) 1577a 1563 (20) 1574 (16)

HCN bend 1361 (7) 1390a 1363 (18) 1363 (46)

HNC bend þ CN str þ OCN bend 1229 (117) 1258a 1275 (102) 1308 (77)

HNC bend 1012 (6) 1046a 1044 (3) 1087 (0)

HCNH tors 981 (6) 1021a 992 (11) 1013 (12)

HNCO tors þ HCNH tors 595 (18) 603a 725 (102) 804 (2)

OCN bend þ HNC bend 533 (10) 581b 577 (26) 592 (96)

HNCO tors þ HCNH tors 153 (294) 289a 333 (122) 601 (86)

Thioformamide

N–H anti sym str 3524 (61) 3495c 3511 (121) 3396 (354)

N–H sym str 3374 (80) 3374c 3281 (189) 3259 (297)

CH str 3009 (26) 2965c 2990 (24) 3014 (5)

NH str þ HNC bend 1584 (226) 1599c 1628 (89) 1631 (80)

HCN bend þ NH str þ CH str þ CN str 1412 (145) 1432c 1425 (185) 1449 (214)

HNC bend þ NH str þ SCN bend þ CyS str 1156 (65) 1287c 1145 (24) 1181 (42)

NH str þ HCN bend 1066 (98) 1125c

HCNH tors 908 (49) 942c 949 (66) 998 (39)

NH str þ HNC bend þ CyS str 839 (25) 870 (21) 861 (44)

HNCS tors þ HCNH tors 566 (0) 716 (36) 732 (204)

CN str þ NH bend 413 (1)

HNCS tors þ HCNH tors 223 (249) 439 (188)

Selenoformamide

N–H anti sym str 3521 (74) 3493 (124) 3410 (489)

N–H sym str 3371 (99) 3300 sd 3236 (208) 3233 (240)

CH str 3024 (13) 2890 md 3017 (14) 3030 (3)

HNC bend 1587 (202) 1601 shd 1631 (88) 1647 (94)

HCN bend þ HNC bend þ CN str 1395 (212) 1390 sd 1407 (200) 1428 (214)

CN str þ HCN bend 1269 (183) 1305 sd 1306 (159) 1335 (55)

HNC bend 1077 (17) 1075 md 1122 (18) 1155 (26)

HCNH tors 902 (55) 954 (72) 997 (51)

CySe str 737 (11) 724 (32) 710 (35)

HNCSe tors þ HCNH tors 588 (0) 734 (24) 782 (95)

SeCN bend 357 (2) 397 (32) 410 (70)

HNCSe tors þ HCNH tors 323 (237) 476 (206) 666 (181)

a Ref. [1].b Ref. [3].c Ref. [5].d Ref. [8].


MP2/6-31G(d,p) level and 11.61 kcal mol21 at MP2/6-

31 þ G(d) level of theory. The same tendency is observed

for the monohydrated and tri-hydrated complexes of 2 and

3. The energy differences for the complexes of 3 with one

water molecule are lower than those for the complexes with

three water molecules at all levels of theory. The reverse

tendency is observed for the monohydrated complex of 2 in

comparison with the tri-hydrated complex at

MP2/6-31G(d,p) level of optimization. To take into account

the role of electron correlation at a higher level, single point

calculations at MP4 level using both basis sets were

performed. This leads to decreases in the energy difference

between the A and B forms of 2 and 3 by 1–1.6 kcal mol21

for the isolated molecules and hydrated complexes.

The transition states for the reactions of tautomerization

in gas phase and water solution of thioformamide and

selenoformamide were also located. The structures of the

TS’s of 2 and 3, corresponding to assisted proton transfer in

tri-hydrated complexes are shown in Figs. 4 and 5,

respectively. The computed barriers of tautomerization of

2 and 3 and their monohydrated and tri-hydrated complexes

calculated at MP2/6-31G(d,p) and MP2/6-31 þ G(d) levels

are given in Table 2.

The direct proton transfer is characterized by decreasing

of the reaction barrier in the order: formamide, thioforma-

mide and selenoformamide. This tendency is valid at

all calculation levels of theory used. A similar tendency

is not observed when water-assisted proton transfer is

realized.

The differences between the barriers of tautomerization

of 2 for direct and assisted (monohydrated complex) proton

transfer at MP2/6-31G(d,p) and MP2/6-31 þ G(d) levels

are 18.95 kcal mol21 and 17.95 kcal mol21, respectively.

The addition of a water molecule to the molecule of

selenoformamide reduces the energy barrier at MP2/6-

31G(d,p) and MP2/6-31 þ G(d) levels by 16.28 kcal mol21

and 16.17 kcal mol21, respectively. For the tri-hydrated

complex the proton transfer activation energy is higher than

that for the monohydrated complex of 2 and 3. This

tendency is observed for calculations at MP2/6-31G(d,p)

level for 2 and for 3 at MP2/6-31 þ G(d) level. At the

highest level of calculation, MP4/6-31 þ G(d)//MP2/6-

31 þ G(d), the computed barrier of tautomerization of 2

for the monohydrated complex is higher than that in the tri-

hydrated complex by 1.89 kcal mol21. In contrast, the

activation barrier of assisted proton transfer reaction in

selenoformamide for the monohydrated complex is lower

by 1.35 kcal mol21 than that for the tri-hydrated one.

The MP2/6-31 þ G(d) calculated interatomic distances

for the tautomers of 2 and 3, TS and their tri-hydrated

complexes are presented in Table 3. The available

experimental data for 2A are also included. The calculated

structural parameters of thioformamide are in good agree-

ment with the experimental data [6] obtained by microwave

spectroscopy. All structures are predicted to be planar. The

predicted N1–H4 and N1–H5 bond lengths are increased in

comparison to the experimental data for 2A by 0.0137 A

and 0.0119 A, respectively. The calculated (1.6358 A) CyS

bond length is longer by 0.0020 A in comparison to the one

found by Dapprich and Frenking [28] and by 0.0272 A

in comparison to the experimental one [6]. The calculated

C–N and C–H bond lengths are in excellent agreement with

the experimental data.

The formation of a complex with three water molecules

induces changes in the geometrical parameters of thiofor-

mamide. According to calculations at MP2/6-31 þ G(d)

level the N1–C2 and C2–H6 bonds are shortened by

0.0208 A and 0.0011 A, respectively, while the thionyl

C2yS3 bond is lengthened by 0.0272 A. The N1–H4 and

N1–H5 bonds are also lengthened because of intermole-

cular hydrogen bonding by 0.0093 A and 0.0075 A,

respectively. These changes are more strongly expressed

in the complex of selenoformamide A 1 3H2O (Fig. 5 and

Table 3). The N1–C2 bond is shortened by 0.0228 A and

C2ySe3 bond is lengthened by 0.0329 A.

Unlike formamide where the water molecules are

situated in the plane of the solute, in the cases of

thioformamide and selenoformamide two water molecules

form a cluster which is situated in the plane of the

thioformamide (Fig. 4) and above the plane of the

selenoformamide (Fig. 5). There are intermolecular hydro-

gen bonds O8· · ·H4 and O13· · ·H5 in the tri-hydrated

complexes of 2A and 3A. The O13· · ·H5 distance is shorter

than the O8· · ·H4 distance in the thioformamide complex

while in the selenoformamide complex the reverse is

observed (2A, O8· · ·H4: 1.9679 A, O13· · ·H5: 1.9480 A;

3A, O8· · ·H4: 1.9296 A, O13· · ·H5: 2.0976 A). The calcu-

lated H7· · ·O3, H7· · ·S3 and H7· · ·Se3 distances are 1.9333,

2.4574 and 2.5374 A. Therefore an intermolecular hydrogen

bond exists only in the formamide –water complex.

Probably for this reason the two water molecules outside

the reaction site form a cluster.

The MP2/6-31 þ G(d) calculated distance O11· · ·H15 is

6.7296, 5.4808 and 1.9107 A in the tri-hydrated complexes

A 1 3H2O of formamide, thioformamide and selenoforma-

mide, respectively (see Figs. 3–5) and therefore in the last

case intermolecular H-bonding exists between the two water

molecules.

In Table 4 are given the MP2/6-31 þ G(d) calculated

values for the vibrational frequencies and IR intensities of

molecules 2 and 3 and their hydrated complexes.

The calculated frequencies for thioformamide and seleno-

formamide are in agreement with available experimental

data [5,8]. An analysis of the theoretical spectrum of

thioformamide shows that the CyS stretching vibration is

coupled with in-plane deformation vibrations and a N–H

stretching vibration. The bands for which this vibration

contributes to the other normal modes were calculated to be

at 1156 and 839 cm21. In contrast to them the calculated

CySe stretching vibration (737 cm21) in selenoformamide

is characteristic. Similarly to formamide, hydration shifts

the N–H stretching vibrations to low frequencies, while


the HNC bending vibration shifts to higher frequencies.

However, nCH is calculated to be lower for the mono-

hydrated complex and higher for the tri-hydrated complex in

comparison with the isolated molecule.

The potential energy along the minimum energy path

(MEP) in the tri-hydrated formamide–water complex is

illustrated in Fig. 6. From the examination of the structural

changes from reactant to product it can be concluded that

Fig. 4. Two projections of thioformamide–water complexes. The geometries were optimized at MP2/6-31 þ G(d) level of theory. Mulliken charges (e2) on the

atoms are given.


Fig. 5. Two projections of selenoformamide–water complexes. The geometries were optimized at MP2/6-31 þ G(d) level of theory. Mulliken charges (e2) on

the atoms are given.


the N1–C2–O3 bond angle is compressed by 3.38 from the

equilibrium value and the N1–H4 bond is stretched from an

equilibrium value of 1.0200 A to 1.2989 A at the transition

state. Simultaneously the O8–H7 bond lengthens from

0.9839 A in the equilibrium structure to 1.2921 A at the

transition state. According to the calculated Mulliken

atomic charges (Fig. 3) on the migrated H4 and H7 atoms

the process seems to be proton transfer. The two water

molecules outside the reaction site are situated in the plane

of the formamide in the precursor complex A 1 3H2O, in

the plane of the transition state TS 1 3H2O and in the

plane of the formamidic acid B 1 3H2O, as can be seen

from Fig. 3. However, in the cases of thioformamide and

selenoformamide the two water molecules form a

cluster as mentioned above. In the transition state

TS 1 3H2O this cluster is situated above the plane of the

solute (Figs. 4 and 5).

Following the reaction path the N1 – C2 – S3 and

N1–C2–Se3 angles are compressed by 3.58 and 4.48,

respectively. The N1–H4 bond is stretched by 0.5568 and

0.6154 A for 2 and 3, respectively, while the O8–H7 bond

lengthens by 0.2262 and 0.2536 A for 2 and 3, respectively.

According to the Mulliken atomic charges given in Figs. 4

and 5, proton (H4) migrates from the amino group of

thioformamide and selenoformamide to the oxygen (O8) of

the water molecule and polar hydrogen atom (H7) of the

water molecule migrates to the sulphur or selenium atom of

2 or 3, respectively. This migration process is accompanied

by charge transfer from the reaction site to the cluster of two

water molecules. Figs. 4 and 5 show that there is a total

charge of 20.06 e2 on the cluster in TS 1 3H2O.

4. Conclusions

The water-mediated proton transfer between the amino

and imino tautomeric forms of 1–3 is investigated at the

correlated MP2 and MP4 levels of theory. Basis set and

electron correlation effects are significant. The height of the

proton transfer barrier for the monohydrated complex of the

investigated compounds is approximately 2 times lower

than the corresponding height for the direct proton transfer.

However, the addition of three water molecules, which

models a first hydration shell around formamide and its

chalcogen analogues, leads to increasing of the barrier

height by 1 – 1.5 kcal mol21 at different levels of

calculation. Solvent-assisted proton transfer in the forma-

mide–water system proceeds via a mechanism involving

simultaneous compression of the NCO bond angle,

migration of a proton of the amino group of formamide to

the oxygen of the water molecule and migration of a proton

of the water molecule to the oxygen atom of formamide.

Unlike in formamide, in thioformamide–water and seleno-

formamide–water systems proton transfer occurs from the

amino group of thioformamide/selenoformamide to the

oxygen of the water molecule and migration of a polar

hydrogen atom of the water molecule to the chalcogen atom.

The geometric parameters obtained at MP2/6-31 þ G(d)

level were found to be similar to those at MP2/6-31G(d,p)

level.

Fig. 6. MP2/6-31 þ G(d) calculated IRC profiles of the solvent-mediated

proton transfer reactions in the formamide–water complex (Fig. 3),

thioformamide–water complex (Fig. 4) and selenoformamide–water

complex (Fig. 5). ET in a.u. and IRC in au1/2bohr.


5. Suplementary material available

Optimized geometries of all tautomers, transition states

and hydrated complexes of the compounds studied are

available at http://www.orgchm.bas.bg/labs/tc/venelin/.

Detailed calculated results on assisted proton transfer in

formamide can be found at http://preprint.chemweb.com/

physchem/0305007.

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http://www.orgchm.bas.bg/labs/tc/venelin

http://preprint.chemweb.com/physchem/0305007

http://preprint.chemweb.com/physchem/0305007

http://www.classic.chem.msu.su/gran/gamess/index.html

http://www.classic.chem.msu.su/gran/gamess/index.html

water-assisted proton transfer in formamide, thioformamide and selenoformamide

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