Blue-shifted and red-shifted hydrogen bonds: Theoretical study

of the CH3CHO/NH3 complexes

Yong Yang*, Weijun Zhang, Shixin Pei, Jie Shao, Wei Huang, Xiaoming Gao

Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, P. R. China.

Received 19 May 2005; revised 4 July 2005; accepted 8 July 2005

Available online 1 September 2005

Abstract

Two stable CH3CHO/NH3 complexes with two H-bonds were found by ab initio methods at MP2/6-31G(d), MP2/6-311CG(d,p) and

MP2/6-311CCG(2d,2p) levels, respectively. The complex A exhibits simultaneously red-shifted N–H/O and blue-shifted C–H/N H-

bonds. The complex B possesses simultaneously two red-shifted H-bonds: N–H/O and C–H/N. From the NBO analysis, it becomes

evident that the three red-shifted H-bonds can be explained on the basis of the two opposite effects: hyperconjugation and rehybridization.

The blue-shifted H-bond is a result of conjunct C–H bond strengthening effects of hyperconjugation and rehybridization due to existence of

the significant electron density redistribution effect.

q 2005 Elsevier B.V. All rights reserved.

Keywords: Red-shifted H-bond; Blue-shifted H-bond; AIM topological analysis; NBO analysis

1. Introduction

Hydrogen bond(H-bond) plays a very important role in

chemistry, physics and biology [1–3]. A characteristic

feature of H/Y H-bond formation in an X–H/Y system is

X–H bond lengthening with a concomitant red shift of the

X–H stretching frequency. However, a number of exper-

imental and theoretical studies have reported the existence

of blue-shifted H-bonds[4–10]. Upon its discovery, blue-

shifted H-bonds received much attention from theoreticians

who suggested several explanations for this phenomenon.

Hobza and co-workers have suggested a two-step mechan-

ism that involves Electron Density Transfer (EDT) from the

proton acceptor to the remote part of the proton donor,

which in turn leads to a shortening of the X–H bond[11].

Having a different view, some researchers favor the view

that there are no fundamental differences in the nature of

red-shifted and blue-shifted H-bonds[12–17]. Recently, a

new insight into the nature of red-shifted and blue-shifted

H-bonds was presented by Alabugin et al[18]. The authors

0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2005.07.006

* Corresponding author. Tel.: C86-551-5593174; fax: C86-551-

5591551.

E-mail address: yongyang@aiofm.ac.cn (Y. Yang).

have shown that blue-shifted H-bonds is not aberration but a

consequence of Bent’s rule where an increase in

the s-character of the hybrid orbital of the X–H bond upon

X–H/Y H-bond formation takes place.

Formaldehyde(HCHO) and acetaldehyde(CH3CHO) are

among the most abundant carbonyls in the atmosphere[19].

To the best of our knowledge, no investigations on

the interesting red-shifted and blue-shifted H-bonds in the

CH3CHO/NH3 complexes are performed up to now. In the

present work, we study the H-bonds in the CH3CHO/NH3

complexes. The main purpose of the work is to confirm the

existence of H-bonds by AIM topological analysis and

discuss the origin of red-shifted and blue-shifted H-bonds

by NBO analysis.

2. Computational methods

The structures and harmonic vibration frequencies of the

monomers and complexes were studied using the MP2

method with the 6-31G(d), 6-311CG(d,p) and 6-311CCG(2d,2p) basis sets, respectively. The basis set super-

position error (BSSE) was calculated according to the

counterpoise method proposed by Boys and Bernardi[20].

We performed natural bond orbital(NBO)[21] calculations

for the monomers and complexes at the MP2/6-311CG(d,p)

Journal of Molecular Structure: THEOCHEM 732 (2005) 33–37

www.elsevier.com/locate/theochem

Fig. 1. Optimized geometries of the monomers and the complexes at the

MP2/6-311CG(d,p) level Bond lengths in (A),Bond angles in (o).

Y. Yang et al. / Journal of Molecular Structure: THEOCHEM 732 (2005) 33–3734

level. Atoms in molecules(AIM)[22] analysis is also carried

out at the MP2/6-311CG(d,p) level. All the calculations are

performed using the Gaussian-98 program packages[23].

3. Results and discussion

3.1. Geometries, frequencies and energies

The structures of the complexes A and B are shown in

Fig. 1. The geometric parameters calculated using the MP2/

6-31G(d), MP2/6-311CG(d,p) and MP2/6-311CCG(2d,2p) levels are listed in Table 1. As shown in

Table 1, the results indicate that there is a good agreement

at various levels. For the complex A, the calculated

distances for O6/H9 and N8/H7 are 2.2544A and

2.6939A at the MP2/6-311CG(d,p) level, respectively.

It is worthy of mentioning that there is a C2-O6-H9-N8-H7

five-membered ring in the complex A. In the cyclic

structure, N8-H9/O6 and C2-H7/N8 are not linear.

From Table 1, it can be seen that there is a large blue shift of

the C2-H7 stretching frequency and a large red shift of the

N8-H9 stretching frequency in the complex A. These

frequency shifts correlate with the bond contraction of

the C2–H7 bond and bond elongation of the N8–H9 bond.

Table 1

The change of bond length, the change of bond stretching frequency and the inte

G(d,p) and MP2/6-311CCG(2d,2p) levels

MP2/6-31G(d) M

CH3CHO/NH3 A

Dr(N8–H9)/A C0.0026 CDv(N8–H9)/cmK1 K14, K25 K

Dr(C2–H7)/A K0.0053 K

Dv(C2–H7)/cmK1 C71 C

DEBSSE/kJ molK1 K11.50 KCH3CHO/NH3 B

Dr(N8–H9)/A C0.0023 C

Dv(N8–H9)/cmK1 K13, K22 KDr(C1–H3)/A C0.0005 C

Dv(C1–H3)/cmK1 K5, K8 K

DEBSSE/kJ molK1 K12.33 K

The results indicate that the blue-shifted H-bond (C2–H7/N8) and red-shifted H-bond (N8–H9/O6) happen simul-

taneously in the complex A.

The structure of the complex B is similar to that of

complex A. The corresponding lengths for O6/H9 and

N8/H3 are 2.2823A and 2.5972A in the complex B at the

MP2/6-311CG(d,p) level. The six-membered ring C1–H3–

N8–H9–O6–C2 exists in the complex B. The complex B

exhibits a small red shift of the C1–H3 stretching frequency

and a large red shift of the N8–H9 stretching frequency. The

small red shift of the C1–H3 stretching frequency correlates

with the slight C1–H3 bond elongation and the large red

shift of the N8–H9 stretching frequency correlates with the

large N8–H9 bond elongation. It is interesting to notice that

the C2–H7 bond length in the complex A is shortened and

the C1–H3 bond length in the complex B is elongated.

As shown in the Table 1, the interaction energies with

BSSE correction for the complex A and B are 12.51 and

12.80 kJ/mol at the MP2/6-311CG(d,p) level, respectively.

The results show that the complex B is slightly more stable

than the complex A.

3.2. AIM topological analysis

To confirm the existence of the red-shifted H-bond and

blue-shifted H-bond, we performed AIM topological

analysis. Popelier proposed a set of criteria for the existence

of H-bond[24,25], among which three are most often

applied[26]. The electron density and its Laplacian for the

H/Y contact within the X–H/Y H-bond should have a

relatively high value. Both parameters for closed-shell

interactions as H-bond are positive and should be within the

following ranges: 0.002–0.035 a.u. for the electron density

and 0.024–0.139 a.u. for its Laplacian. This quantity of the

electron density of the BCP (Bond Critical Point) is denoted

by r and is listed in Table2. For the complex A, the values of

the electron density r for N8/H7 and O6/H9 are 0.0076

and 0.0131 a.u., respectively. The values for N8/H3 and

O6/H9 in the complex B are 0.0083 and 0.0120 a.u.,

respectively. These values do fall within the proposed

raction energy with BSSE correction at the MP2/6-31G(d), MP2/6-311C

P2/6-311CG(d,p) MP2/6-311CCG(2d,2p)

0.0035 C0.0038

24, K36 K26, K38

0.0035 K0.0031

50 C46

12.51 K14.06

0.0033 C0.0036

24, K34 K25, K36

0.0003 C0.0008

4, K7 K8, K11

12.80 K14.20

Table 2

Topological parameters of the bond critical point at the MP2/6-311CG(d,p) level

BCP r P2r l1 l2 l3 3

CH3CHO/NH3 A

O6–H9 0.0131 0.0491 K0.0141 K0.0138 0.0770 0.0235

N8–H7 0.0076 0.0279 K0.0069 K0.0037 0.0385 0.8647

CH3CHO/NH3 B

O6–H9 0.0120 0.0443 K0.0128 K0.0128 0.0699 0.0019

N8–H3 0.0083 0.0262 K0.0076 K0.0061 0.0399 0.2355

Y. Yang et al. / Journal of Molecular Structure: THEOCHEM 732 (2005) 33–37 35

typical range of the H-bond. According to the AIM

topological analysis, the r is used to describe the strength

of a bond. In general, the larger the value of r, the stronger

the bond. Since the r(N/H) is smaller than the r(O/H) in

both of the complexes, we expect the former bond to be

weaker than the latter. The ellipticity 3 is defined as l1/l2

K1 and measures the extent to which charge is preferen-

tially accumulated. The ellipticity provides a measure for

not only the p character of a bond but also its structural

stability. Substantial bond ellipticities reflect structural

instability, that is, the bond can easily be ruptured. In

Table 2, we see that 3(N/H) is much larger than 3(O/H),

confirming that the former bond is weaker. The P2rdescribes the characteristic of the bond. Where P2r ! 0,

the bond is covalent bond, as P2r O 0, the bond belongs to

the ionic bond, hydrogen bond and van der Waals

interactions. Here we are only concerned with the values

of P2r for N/H and O/H bonds, which are listed in the

Table 2. The values of P2r for N/H and O/H bonds in

the complexes A and B are in the range of the H-bond.

On the basis of the AIM topological analysis, we have

proved that the C–H/O and N–H/O bonds can be

classified as H-bonds in the complexes A and B. It should be

pointed out that the C–H bond in the complex B exhibits a

red shift whereas the C–H bond in the complex A exhibits a

blue shift. However, the AIM analysis does not reveal the

origin of the red-shifted and blue-shifted H-bonds. This

problem was solved by performing the Natural Bond

Orbital(NBO) analysis.

Table 3

The energy of hyperconjugative interaction (kJ/mol) and occupancy (e) at

the MP2/6-311CG(d,p) level

E(2) Monomer CH3CHO/NH3 A

CH3CHO/NH3 B

n1(O6) /s* (N8–H9) – 0.84 1.80

n2(O6) /s* (N8–H9) – 6.95 5.73

n(N8) /s* (C1–H3) – – 5.94

n(N8) /s* (C2–H7) – 2.43 –

n2(O6) /s* (C2–H7) 122.26 109.66 119.08

Occupancy

s* (N8–H9) 0.0 0.00355 0.00296

s* (C1–H3) 0.00491 0.00499 0.00687

s* (C2–H7) 0.05237 0.04771 0.05074

n1(O6) 1.98501 1.98484 1.98431

n2(O6) 1.91373 1.91801 1.91606

3.3. NBO analysis

It is clear from Fig. 1 that the H-bonds are complicated in

the complexes A and B. The complex A exhibits

simultaneously red-shifted N–H/O and blue-shifted

C–H/N H-bonds. The complex B possesses two red-

shifted H-bonds. To interpret and explain the nature of the

above-mentioned red-shifted and blue-shifted H-bonds, we

performed the NBO analysis for the complexes A and B.

The results are presented in Table 3 and 4. In the NBO

analysis, the importance of hyperconjugation interaction

and Electron Density Transfer (EDT) from lone electron

pairs of the Y atom to the X–H antibonding orbital in the

X–H/Y system is well-documented[21]. In general, such

interactions lead to an increase in population of X–H

antibonding orbital, which elongates the X–H bond. On the

basis of the theoretical analysis, it should not be difficult to

explain the red shift of N–H bond stretching frequency in

the complexes A and B. For the N8–H9/O6 H-bond in the

complex A, we note that the E(2) for n1(O6) /s* (N8–H9)

is relatively large. The corresponding increase of electron

density in the s* (N8–H9) weakens and elongates the

N8–H9 bond, which results in the red shift of the N8–H9

bond stretching frequency. From Table 3, it can be seen that

similar results are obtained in the complex B. To this point,

the NBO results fully explain the observations of the red

shift of the N–H bond in the complexes A and B.

We pay more attention to the changes of the C–H bond

stretching frequency. It is worth pointing out that there is an

evident difference for the changes of the C–H bond

stretching frequency in the complexes A and B. The

existence of large blue-shifted C–H bond in the complex A

is surprising because the magnitude of E(2) for n(N8) /s*

(C2–H7) is significant. This interaction should lead to the

increase of electron density in the s* (C2–H7), which

weakens the C2–H7 bond associated with elongation.

However, the opposite is found. From Table 3, it can be

seen that the electron density in the s* (C2–H7) decreased

which unambiguously meant strengthening and contraction

of the C2–H7 bond and a blue shift of the stretching

frequency. The question is how to explain the electron

density decrease in the s* (C2–H7). We will show below

that the electron density redistribution plays a significant

role for the C2–H7 bond contraction. It can be seen in

Table 3 that there a significant decrease of E(2) for n2(O6)

/s* (C2–H7) in the complex A, relative to the monomer

CH3CHO. A decrease in the n2(O6) /s* (C2–H7)

interaction leads to a decrease in the s* (C2–H7)

population. In addition, it should be point out that electron

Table 4

The natural atomic charges (e), hybridization and polarization at the

MP2/6-311CG(d,p) level

Monomers CH3CHO/NH3 A

CH3CHO/NH3 B

q(O6) K0.6160 K0.6454 K0.6429

q(H3) 0.2047 0.2032 0.2360

q(H7) 0.0893 0.1206 0.0902

q(N8) K1.0298 K1.0576 K1.0577

q(H9) 0.3433 0.3718 0.3745

spn(C1–H3) sp2.96 sp2.96 sp2.81

% s-char 25.17% 25.18% 26.18%

spn(C2–H7) sp2.32 sp2.24 sp2.33

% s-char 30.06% 30.75% 29.96%

spn(N8–H9) sp2.86 sp2.72 sp2.72

% s-char 25.85% 26.80% 26.82%

pol C1%

(sC1–H3), H3%

60.31%

39.69%

60.24%

39.76%

61.94%

38.06%

pol C2%

(sC2–H7), H7%

55.83%

44.17%

57.30%

42.70%

55.84%

44.16%

pol N8%

(sN8–H9), H9%

67.22%

32.78%

68.81%

31.19%

68.91%

31.09%

Y. Yang et al. / Journal of Molecular Structure: THEOCHEM 732 (2005) 33–3736

density of n2(O6) should have a significant decrease due to

the large interactions from n2(O6) to s* (N8–H9). However,

it can be seen that electron density of n2(O6) has a

significant increase from Table 3. On the basis of these

analyses, the surprising phenomenon that the electron

density of n2(O6) has a significant increase while s*

(C2–H7) has an evident decrease can well be interpreted.

The situation in the complex B is different from the complex

A. The electron density redistribution is not important for

C1–H3 bond in the complex B. The dominant part of the

electron density transfer from n(N8) is directed to the s*

(C1–H3). The corresponding increase of electron density in

the s* (C1–H3) causes weakening of the C1–H3 bond.

It should be pointed out that CHO and CH3 groups in the

CH3CHO have a significant difference. CHO group contains

an O atom. It seems reasonable for us to assume CHnXm

group containing X atoms (X possesses one or more lone

electron pairs) is a necessary condition for the existence of

this significant electron density redistribution. When the

hyperconjugation interaction is weak and electron density

redistribution effect is significant, we would expect electron

density decrease in the C–H antibonding orbital. The result

leads to the C–H bond contraction and blue shift of the C–H

stretching frequency.

Recently, “rehybridization” model was presented by

Alabugin et al[18]. The authors have shown that this X–H

bond strengthening effect is an increase in s-character of

carbon hybrid orbital in the X–H bond. The increase in

s-character is a direct consequence of Bent’s rule.

According to Bent’s rule, atoms tend to maximize the

amount of s-character in hybrid orbitals aimed toward

electropositive substituents and direct hybrid orbital with

the larger amount of p-character toward more electro-

negative substituents. A direct consequence of Bent’s rule

which is important for H-bonds is that a decrease in

effective electronegativity of hydrogen in an X–H bond

leads to an increase in the s-character of the hybrid orbital of

this bond. Such a decrease in effective electronegativity

leads to increasing bond polarization.

Table 4 shows the changes of natural atomic charges,

polarization and hybridization in the complexes A and B.

Not surprisingly, the changes in the electronic structure of

the complex A are an increase in positive charge on H7 atom

and a simultaneous increase in negative charge on O6 atom,

which lead to the increase in the C2–H7 bond polarization.

These changes result in a simultaneous increase in the

s-character in the carbon hybrid orbital of C2–H7 bond,

which leads to C2–H7 bond contraction. Similarly, the

negativity of the O6 atom increases and the positivity of the

H3 atom increases in the complex B. The C1–H3 bond

becomes more polarized. The s-character of C1–H3 bond

increased which strengthens the C1–H3 bond.

To summarize, the hyperconjugation interactions are

relatively large and electron density redistribution effects are

not important for the N8–H9/O6 in the complex A and

C1–H3/N8 and N8–H9/O6 in the complex B. These

results lead to an increase in population of an X–H(XZC, N)

antibonding orbital, which elongates the X–H bond. The

X–H bond strengthening effect is an increase in s-character

in the X hybrid orbital of X–H bond. For the three H-bonds,

the mechanism is combination of the two effects: hypercon-

jugation X–H bond weakening and rehybridization X–H

bond strengthening. Because hyperconjugation and rehy-

bridization act in opposite directions, the red shift and blue

shift of the bond X–H is a result of a balance of the two

effects. Then, the fact is simple: the hyperconjugation is

dominant and overshadows the rehybridization in the three

H-bonds. Therefore, it is not surprising that the three H-bonds

exhibit the bond elongation and the red shift of stretching

frequency.

A different situation exists for C2–H7 blue shift in the

complex A. Here the situation is more complicated. It is

important to further emphasize the fact: when the

hyperconjugation interaction is weak and the electron

density redistribution effect is significant, the hyperconjuga-

tion can be inhibited and be considered even as the X–H

bond strengthening effect. Then, hyperconjugation and

rehybridization act in uniform directions in the C2–H7

bond. The C2–H7 bond contraction and blue shift of

stretching frequency is a result of conjunct C2–H7 bond

strengthening effects.

4. Conclusions

Two CH3CHO/NH3 complexes were found to be

stable. The complex A exhibits simultaneously red-shifted

N–H/O and blue-shifted C–H/N H-bonds. The complex

B possesses simultaneously two red-shifted H-bonds:

N–H/O and C–H/N. From the NBO analysis, it becomes

evident that the three red-shifted H-bonds can be explained

Y. Yang et al. / Journal of Molecular Structure: THEOCHEM 732 (2005) 33–37 37

on the basis of the two opposite effects: hyperconjugation

and rehybridization. For the blue-shifted H-bond, NBO

analysis fully interprets the electron density decrease of the

s* (C–H) due to existence of the significant electron density

redistribution effect. When the hyperconjugation interaction

is weak and the electron density redistribution effect is

significant, the hyperconjugation can be considered as the

C–H bond strengthening effect. We conclude that the

unusual blue-shifted H-bond is due to an intricate

combination of two strengthening effects of hyperconjuga-

tion and rehybridization.

Acknowledgements

This work is supported by the National Natural Science

Foundation of China (G20477043) and Knowledge Creative

Program by Chinese Academy of Sciences (KJCX2-SW-

H08). The authors express our gratitude to the referees for

their value comments.

References

[1] S. Scheiner, Hydrogen Bonding, Oxford University Press, New York,

1997.

[2] G.A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford

University Press, New York, 1997.

[3] G.R. Desiraju, T. Steiner, The Weak Hydrogen Bond, Oxford

University Press, New York, 1999.

[4] M. Budesinsky, P. Fiedler, Z. Arnold, Synthesis (1989) 858.

[5] S.N. Delanoye, W.A. Herrebout, B.J. van der Veken, J. Am. Chem.

Soc. 124 (2002) 11854.

[6] P. Hobza, V. Spirko, Z. Havlas, K. Buchhold, B. Reimann, H.-D.

Barth, B. Brutschy, Chem. Phys. Lett. 299 (1999) 180.

[7] B. Reimann, K. Buchhold, S. Vaupel, B. Brutschy, Z. Havlas,

V. Spirko, P. Hobza, J. Phys. Chem. A 105 (2001) 5560.

[8] P. Hobza, Z. Havlas, Chem. Phys. Lett. 303 (1999) 447.

[9] P. Hobza, V. Spirko, H.L. Selzle, E.W. Schlag, J. Phys. Chem. A 102

(1998) 2501.

[10] J. Chocholousova, V. Spirko, P. Hobza, Phys. Chem. Chem. Phys. 6

(2004) 37.

[11] P. Hobza, Z. Havlas, Chem. Rev. 100 (2000) 4253.

[12] Y. Gu, T. Kar, S. Scheiner, J. Am. Chem. Soc. 121 (1999) 9411.

[13] S. Scheiner, S.J. Grabowski, T. Kar, J. Phys. Chem. A 105 (2001)

10607.

[14] S. Scheiner, T. Kar, J. Phys. Chem. A 106 (2002) 1784.

[15] X. Li, L. Liu, H.B. Scheiner, J. Am. Chem. Soc. 124 (2002)

9636.

[16] K. Hermansson, J. Phys. Chem. A 106 (2002) 4695.

[17] A. Masunov, J.J. Dannenberg, R. Contreras, J. Phys. Chem. A 105

(2001) 4737.

[18] I.V. Alabugin, M. Manoharan, S. Peabody, F. Weinhold, J. Am.

Chem. Soc. 125 (2003) 5973.

[19] B. D’Anna, V. Bakken, J.A. Beukes, C.J. Nielsen, K. Brudnik,

J.T. Jodkowski, Phys. Chem. Chem. Phys. 5 (2003) 1790.

[20] S.F. Boys, F. Bernardi, Mol. Phys. 19 (1970) 553.

[21] A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1988)

899.

[22] R.F.W. Bader, Atoms in Molecules: a Quantum Theory, Oxford

University Press, Oxford, 1990.

[23] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,

J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, Jr., R.E.

Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels,

K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi,

R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J.

Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K.

Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski,

J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P.

Piskorz, I. Komaromo, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith,

M.A. Allaham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M.

Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.M. Wong, J.L.

Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople,

Gaussian 98, Revision A.7, Gaussian, Inc., Pittsburgh PA, 1998.

[24] U. Kock, P.L.A. Popelier, J. Phys. Chem. 99 (1995) 9747.

[25] P.L.A. Popelier, J. Phys. Chem. A 102 (1998) 1873.

[26] P. Lipkowski, S.J. Grabowski, T.L. Robinson, J. Leszczynski, J. Phys.

Chem. A 108 (2004) 10865.