Intermolecular hydrogen bonding between N-substituted caproamides and tetrahydrofuran

Article abstract of DOI:10.1134/S0022476613020212

The results are reported of a study of hydrogen bonding between various N-substituted caproamides and tetrahydrofuran as an O-electron donor by means of FTIR spectroscopy. The spectroscopic characteristics for N-H.O hydrogen bonded complexes are given. The B3LYP functional with 6-31G**basis set has been used to calculate the structural parameters of the studied hydrogen bonded complexes. It can be assumed that both inductive and steric effects play an important role in the stability of these hydrogen bonded complexes.

Full text of DOI:10.1134/S0022476613020212

Journal of Structural Chemistry. Vol. 54, No. 2, pp. 431-436, 2013
Original Russian Text Copyright © 2013 by B. Jović, A. Nikolić, B. Holló
BRIEF COMMUNICATIONS
INTERMOLECULAR HYDROGEN BONDING BETWEEN
N-SUBSTITUTED CAPROAMIDES AND TETRAHYDROFURAN
©
B. Jović, A. Nikolić, and B. Holló
UDC 541.6:541.49:546.17:546.21
The results are reported of a study of hydrogen bonding between various N-substituted caproamides and
tetrahydrofuran as an O-electron donor by means of FTIR spectroscopy. The spectroscopic characteristics
for N–H⋯O hydrogen bonded complexes are given. The B3LYP functional with 6-31G** basis set has
been used to calculate the structural parameters of the studied hydrogen bonded complexes. It can be
assumed that both inductive and steric effects play an important role in the stability of these hydrogen
bonded complexes.
DOI: 10.1134/S0022476613020212
Keywords: hydrogen bonding, N-substituted caproamide, tetrahydrofuran, DFT.
INTRODUCTION
Hydrogen bonding has a great importance in chemistry and biology. Numerous theoretical and experimental studies
have been devoted to hydrogen bonding [1-3]. Knowledge of N–H⋯O interactions of N-substituted amides contributes to a
better understanding of the structure of polypeptides and proteins [4]. In our laboratory, during continuous and extensive
research of N-substituted amides and their interactions in solutions, N–H⋯O, [5-8] hydrogen bonded complexes were
investigated using IR techniques. The application of the B3LYP density functional with various basis sets can provide
reasonable information about the geometry and energy of different hydrogen bonded complexes [9, 10]. In the present work,
we investigated the N–H⋯O interactions of newly synthesised N-alkyl caproamides in order to provide better insight into the
nature of the hydrogen bonding of secondary amides.
EXPERIMENTAL
3 2 4
Various N-monosubstituted caproamides (NsCA) of general formula CH (CH ) CONHR, wherein R is n-propyl, n-
butyl, sec-butyl, iso-butyl, tert-butyl, and n-pentyl (NPrC, NBuC, NsBC, NiBC, NtBC, NPeC), were synthesised by
Schotten-Baumann reaction, by acylation of the corresponding amines with alkyl chloride. The purity of these N-
monosubtituted amides was checked by GC and mass spectrometry. Based on the GC results all the samples used in this work
had purities 99.2% or better. Tetrahydrofuran was obtained from commercial sources (Fluka, > 97%) and was used without
further purification.
Department of Chemistry, Faculty of Sciences, University of Novi Sad, Novi Sad, Serbia;
branislav.jovic@dh.uns.ac.rs. The text was submitted by the authors in English. Zhurnal Strukturnoi Khimii, Vol. 54, No. 2,
pp. 369-374, March-April, 2013. Original article submitted November 25, 2011; revised May 21, 2012.
431
0022-4766/13/5402-0431

Fig. 1. IR spectra of N-substituted caproamides in the presence of tetrahydrofuran: N-propyl caproamide (a), N-
butyl caproamide (b), N-iso-butyl caproamide (c), N-sec-butyl caproamide (d ), N-tert-butyl caproamide (e), N-
pentyl caproamide (f ).
–3
In order to avoid self-association, amide concentrations in carbon tetrachloride solutions were below 0.003 mol⋅dm .
–3 –3
The concentration of tetrahydrofuran varied between 0.25 mol⋅dm and 1 mol⋅dm .
Infrared spectra were obtained using a Thermo-Nicolet Nexus 670 instrument. A DTGS detector was employed in
IR measurements. Samples were placed in 1-5 cm (IR) UVIRSIL cells and the measurements were performed at 298 K. The
–1
–1
reported frequencies and half-widths were reproducible within 0.2 cm and 1 cm , respectively. The integrated molar
absorption coefficients were obtained within ± 5%. The equilibrium constants were determined with average relative standard
deviation of 6.6%. The formation constants were determined from the monomer absorbancy using Becker’s procedure [11].
Quantum chemical calculations. Density functional theory with the B3LYP functional [12, 13] was used in
combination with the 6-31G** basis set. The Berny algorithm was used for the optimization of the molecular geometry
expressed in terms of the bond lengths and valence angles. Standard convergence criteria were used to complete the geometry
optimization process. All of the calculations were performed with the Gaussian 09 software package [14].
RESULTS AND DISCUSSION
Fig. 1 relates to the IR spectra of NsCA in the presence of tetrahydrofuran. The spectroscopic parameters for N–
H⋯O hydrogen bonded complexes obtained by the IR measurements are summarized in Table 1. Fig. 2 shows a graphical
determination of the equilibrium constants on the basis of the Beceker method. The equilibrium constant values and accuracy
parameters for the equilibrium constants determination are given in Table 2. In order to evaluate the contribution of the
charge transfer mechanism, the intermolecular transition moments were calculated from the difference in the integrated
intensities for the free and hydrogen bonded N–H groups [15], and are also presented in Table 1.
2
∂μ
3000mc
t
0
0
=
B
hb
−
B
,
(1)
(
f
)
∂r
NAπ
0
0
where Bhb and B are the integrated intensities of the bands of hydrogen bond and free NH groups; m is the reduced mass of
f
432

Fig. 2. Geometrically optimized hydrogen bonded complexes of N-substituted caproamides with tetrahydrofuran
N-propyl caproamide (a), N-butyl caproamide (b), N-iso-butyl caproamide (c), N-sec-butyl caproamide (d ), N-
tert-butyl caproamide (e), N-pentyl caproamide (f ).
TABLE 1. Spectroscopic Parameters for N–H…O Hydrogen Bonded Complexes Obtained in IR Measurements
–1
–1
–3 2
ε⋅10 , cm ⋅mol
–1
0 –6
B ⋅10 , cm⋅mol
–1
20
(∂μ /∂r)×10 , c
Δν, cm
1/2
ν , cm
t
NPrC–THF
NBuC–THF
NiBC–THF
NsBC–THF
NtBC–THF
NPeC–THF
94.1
94.2
94.7
91.6
85.2
94.2
55
55
57
51
46
53
130.6
166.1
80.1
10.6
13.6
6.2
2.36
2.86
1.38
2.35
2.14
3.09
124.9
125.4
150.4
8.5
7.6
11.4
TABLE 2. The Equilibrium Constants Obtained in IR Measurements, Determination Coefficients and Standard Deviations
–1
K, dm⋅mol
2
–1
2
R
SD
K, dm⋅mol
R
SD
NPrC–THF
NBuC–THF
NiBC–THF
0.73
0.61
1.11
0.9907
0.9764
0.9795
0.057
0.052
0.052
NsBC–THF
NtBC–THF
NPeC–THF
0.69
0.47
0.59
0.9981
0.9915
0.9863
0.023
0.051
0.027
the NH oscillator; c is the velocity of light; and NA is the Avogadro number.
The obtained IR spectroscopic parameters clearly indicate N–H⋯O hydrogen bonding between N-substituted
caproamides and the tetrahydrofuran. Comparison of the equilibrium constants as well as the frequency shift values for the
series of examined amides shows that the most stable complex with tetrahydrofuran is formed by N-isobutyl caproamide
while the least stable complex is formed by N-tert butyl caproamide.
In the case of a significant contribution by the charge transfer mechanism, for the series of proton donors, a positive
correlation between the intermolecular transition moments and relative frequency shift can be observed, according to the
433

Fig. 3. Plot of log K298 versus Taft steric, E
and inductive σ* susbstituent constants
bstituent constants.
S
equation [15]
∂μ
Δν
t
=
Q
.
(2)
∂r
ν
0
For the series of amides examined in this study, a correlation between the intermolecular transition moments and
relative frequency shift, which would indicate a contribution from the charge transfer mechanism, was not observed.
The stability of hydrogen bond complexes increases in the order: NtBC < NPeC < NBuC < NsBC < NPrC < NiBC.
Differences in the stability of the N–H⋯O complexes for the six studied N-substituted caproamides can be interpreted using
the Taft equation [16]
0
log K = log K + ρσ* + δE ,
(3)
298
298
S
0
where log K298 represents the formation constant of N-methyl capromaide, σ* and E
for alkyl groups. If log K298 is correlated with both σ* and E a good correlation is obtained
log K = 0.1145 + 3.537σ*−0.3956E , R = 0.975.
represent inductive and steric factors
S
S
(4)
298
S
Good correlation with both σ* and E
S
, indicates that both inductive and steric effects are important in hydrogen
bonded complex formation. Several investigators [17-19] have obtained eqilibrium constants for hydrogen bonding between
phenols with various (cyclic and acyclic) ethers and have established that donor-acceptor interactions are sensitive to both
steric and electronic effects.
The equilibrium constants for N-methyl formamide, N-methyl propionamide, N-methyl trichloroacetamide-
tetrahydrofuran complexes [6, 8, 20] have higher values than the constants obtained in this study.
The geometrically optimized hydrogen bonded complexes are shown in Fig. 4. The structural parameters of the
hydrogen bonded complexes computed at the DFT (B3LYP)/6-31G** level are presented in Table 3.
The correlation coefficients between the parameters obtained by FTIR spectroscopy and DFT calculations are presented
in Table 4. The significance of the reported correlation coefficients was tested with a t test at significance level P < 0.01.
Fig. 5 relates to the linear correlation of frequency shift Δν (N–H) with the N–H⋯O distance (a), H⋯O distance (b).
As can be seen from Table 4, the experimental parameter which most significantly correlates with the theoretical
parameters is the frequency shift Δν (N–H). It is well-known that the frequency shift is negatively correlated with the length
and positively with the strength of the hydrogen bond [21]. Correlations of frequency shift with experimentally and
theoretically obtained geometrical parameters were found for a number of different hydrogen bonded complexes [22-24].
Stronger hydrogen bonds are shorter and cause larger red shifts in the IR spectra.
434

Fig. 4. Graphical determination of the equilibrium constants on the basis of the Beceker method: N-propyl
caproamide (a), N-butyl caproamide (b), N-iso-butyl caproamide (c), N-sec-butyl caproamide (d ), N-tert-butyl
caproamide (e), N-pentyl caproamide (f ), CPA — concentration of proton acceptor THF
Fig. 5. Linear correlation of the frequency shift Δν (N–H) with the N–
H⋯O distance (a), and the H⋯O distance (b).
TABLE 3. Selected Geometrical Parameters Obtained from the (B3LYP/6-31G**) Calculations for the
Optimized Structure of the N-Substituted Caproamide Complexes
C–N
N–H
r(H⋯O)
distance, Å
r(N–H⋯O),
distance, Å
α(N–H⋯O)
angle, deg
distance, Å
distance, Å
NPrC–THF
NBuC–THF
NiBC–THF
NsBC–THF
NtBC–THF
NPeC–THF
1.362
1.362
1.362
1.361
1.361
1.362
1.015
1.015
1.015
1.016
1.016
1.015
2.001
2.002
1.999
2.005
2.033
2.004
3.015
3.017
3.014
3.021
3.049
3.019
174.9
176.7
176.2
175.1
172.6
176.6
CONCLUSIONS
In this work FTIR spectroscopic and DFT theoretical parameters were obtained for hydrogen bonds of six
investigated N-substituted caproamides with tetrahydrofuran. Correlations between the experimental and theoretical
435

TABLE 4. Correlation Coeffitients Between Spectroscopic and Theoretical Hydrogen Bond Parameters
r(C–N)
r(N–H)
r(H⋯O)
r(N–H⋯O)
α(N–H⋯O)
Δν
0.832
0.856
0.118
0.456
0.422
0.153
–0.832
–0.856
–0.118
–0.456
–0.422
–0.153
–0.977*
–0.928
0.024
–0.982*
–0.935
0.018
0.914
0.824
0.202
0.480
0.418
0.294
ν
1/2
ε
B
K
–0.315
–0.613
–0.067
–0.324
–0.612
–0.074
t
∂μ /∂r
parameters were observed. Based on the obtained data and the correlations, it can be concluded that for the series of
examined caproamides, there is no significant influence from the charge transfer mechanism on the stability of the
complexes. It can be assumed that both inductive and steric effects play an important role in the stability of these hydrogen
bonded complexes.
This work was financially supported by the Ministry of Education and Science of Serbia (Projects Nos. 172013 and
172014).
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