Spectroscopic evidence for an intramolecular C-H···N hydrogen bond: Infrared and Raman spectroscopy and ab initio molecular orbital calculations of N,N-dimethylpropylamine and propyltrimethylammonium bromide

Article abstract of DOI:10.1016/S0022-2860(97)00195-6

The infrared and Raman spectra were measured for N,N-dimethylpropylamine (DPA) in an argon matrix, the liquid and glassy states and aqueous solution, and for propyltrimethylammonium bromide (PTAB) in the solid state and aqueous solution. The ab initio molecular orbital calculations were performed on possible conformers of DPA and propyltrimethylammonium ion. In an argon matrix, the conformational stability of DPA is in the order G'G > GT > TT, which agrees with the prediction by ab initio calculations. The experimentally confirmed high stability of the G'G conformer indicates that an appreciably strong attractive interaction occurs between the methyl hydrogen atom and the nitrogen atom. This interaction may be called the intramolecular 1,4-C-H···N hydrogen bonding. In the liquid and glassy states, the GT conformer is the most stable. In aqueous solution of DPA, the GT and TT conformers are more stabilized than in the neat liquid, possibly because of the formation of more significant intermolecular hydrogen bonds between the nitrogen atom of DPA and water molecules. For PTAB, only the T conformer exists in the solid state, and this conformer is more stable than the G conformer in aqueous solution. The GG and TG conformers of DPA and the G conformer of PTAB are significantly less stabilized by steric repulsions between the CH₃(N) and (C)CH₃ groups.

Full text of DOI:10.1016/S0022-2860(97)00195-6

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Journal of Molecular Structure 435 (1997) 219–228
…
Spectroscopic evidence for an intramolecular C–H N hydrogen bond:
infrared and Raman spectroscopy and ab initio molecular orbital
calculations of N,N-dimethylpropylamine and
propyltrimethylammonium bromide
Keiichi Ohno*, Akimitsu Tonegawa, Hiroshi Yoshida, Hiroatsu Matsuura
Department of Chemistry, Faculty of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739, Japan
Received 10 March 1997; accepted 11 April 1997
Abstract
The infrared and Raman spectra were measured for N,N-dimethylpropylamine (DPA) in an argon matrix, the liquid and
glassy states and aqueous solution, and for propyltrimethylammonium bromide (PTAB) in the solid state and aqueous solution.
The ab initio molecular orbital calculations were performed on possible conformers of DPA and propyltrimethylammonium
ion. In an argon matrix, the conformational stability of DPA is in the order GЈG Ͼ GT Ͼ TT, which agrees with the prediction
by ab initio calculations. The experimentally confirmed high stability of the GЈG conformer indicates that an appreciably strong
attractive interaction occurs between the methyl hydrogen atom and the nitrogen atom. This interaction may be called the
…
intramolecular 1,4-C–H N hydrogen bonding. In the liquid and glassy states, the GT conformer is the most stable. In aqueous
solution of DPA, the GT and TT conformers are more stabilized than in the neat liquid, possibly because of the formation of
more significant intermolecular hydrogen bonds between the nitrogen atom of DPA and water molecules. For PTAB, only the T
conformer exists in the solid state, and this conformer is more stable than the G conformer in aqueous solution. The GG and TG
conformers of DPA and the G conformer of PTAB are significantly less stabilized by steric repulsions between the CH3(N) and
(C)CH3 groups. ᭧ 1997 Elsevier Science B.V.
…
Keywords: Intramolecular C–H N hydrogen bond; Conformation; Infrared and Raman spectra; N,N-Dimethylpropylamine;
Propyltrimethylammonium bromide
…
1. Introduction
bonding of the type A–H B, where A is an atom
such as oxygen and nitrogen being more electro-
negative than hydrogen, and B is an electron donor
such as oxygen, nitrogen and halogens [1–3]. Other
important interactions, which are attractive and may
be regarded as hydrogen bonding, are those of the
Intramolecular and intermolecular interactions play
an important role in the structural stabilization of
molecules in many chemical and biological systems.
One of the most important interactions is hydrogen
…
…
types C–H O and C–H N. In 1962, Sutor
…
proposed the existence of a C–H O hydrogen
* Corresponding author. Fax: +81 824 24 7407; e-mail:
kohno@sci.hiroshima-u.ac.jp
bond [4,5]. Taylor and Kennard showed later the
0022-2860/97/$17.00 ᭧ 1997 Elsevier Science B.V. All rights reserved
PII S0022-2860(97)00195-6

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220
K. Ohno et al./Journal of Molecular Structure 435 (1997) 219–228
crystallographic evidence for an intermolecular
co-addition of 200 scans at a resolution of 1 cm−1. The
matrix-isolation infrared spectra were recorded on a
JEOL JIR-40X Fourier transform spectrophotometer
equipped with a TGS detector by co-addition of 100
scans at a resolution of 1 cm−1. Mixed gas of Ar/DPA
= 1000 was slowly deposited onto a CsI plate kept at
11 K by an Iwatani CryoMini D105 refrigerator.
The Raman spectra were measured for DPA in the
liquid and glassy states and in 30% aqueous solution,
and for PTAB in the solid state and in 30% aqueous
solution on a JEOL JRS-400D spectrophotometer
with an NEC GLG3200 argon ion laser. The Raman
spectra of DPA in the glassy state were measured for
the sample contained in an ampoule held on a copper
block cooled with liquid nitrogen.
…
C–H O hydrogen bond and possible existence of a
…
C–H N hydrogen bond [6]. Desiraju has further
…
studied the C–H O hydrogen bond in crystals [7,8].
The conformational stability of the OC–CC bond
in dialkyl ethers and alcohols has been studied
extensively by experimental and theoretical methods
[9–14]. These studies have shown that the gauche
conformation for this bond is as stable as, or more
stable than, the trans conformation. This conforma-
tional stabilization is explained by an intramolecular
…
CH O interaction, which may be called more
…
specifically the intramolecular 1,4-CH O inter-
action. Another important interaction is the intra-
…
molecular 1,5-CH O interaction, which stabilizes
the gauche–gaucheЈ conformation of the OC–C–OC
bonds in 1,2-dimethoxyethane and its derivatives
[15–23].
3. Calculations
…
On the analogy of the intramolecular CH O inter-
…
action, an intramolecular CH N interaction is antici-
Ab initio MO calculations were performed on DPA
and propyltrimethylammonium (PTA) ion by the
restricted Hartree–Fock method using the 6-31G**
basis set. The energies, molecular geometries and
vibrational wavenumbers were calculated for all of
the possible conformers of these compounds. The cal-
culated wavenumbers were scaled by a factor of 0.93
in the region below 1030 cm−1 for DPA or below
1040 cm−1 for PTA and by a factor of 0.90 in the
region above 1030 or 1040 cm−1 up to 1500 cm−1,
where the vibrations are associated primarily with
the methyl and methylene deformations. These values
of the scale factor were chosen in the light of the
results of the calculations on n-pentane and n-hexane
[26]. The computation in this work was carried out
with the GAUSSIAN 94 program [27] at the Informa-
tion Processing Center of Hiroshima University. The
input data for GAUSSIAN 94 were prepared by a
molecular modeling and molecular graphics program,
MOLDA for Windows [28].
pated to occur in relevant compounds. To study the
…
possible existence of the intramolecular 1,4-CH
N
interaction, we have investigated in this work the con-
formational stability of N,N-dimethylpropylamine
(DPA), (CH3)2NCH2CH2CH3, by infrared and
Raman spectroscopy and ab initio molecular orbital
(MO) calculations. The conformation of a structurally
related compound, propyltrimethylammonium bromide
(PTAB), Br(CH3)3NCH2CH2CH3, was also studied
for comparison.
2. Experimental
DPA was prepared by mixing dimethylamine with
sodium hydrogencarbonate and 1-bromopropane [24].
The product was purified by distillation and the purity
was checked by gas chromatography. PTAB was
prepared by reacting trimethylamine in ethanol with
1-bromopropane [25]. The product was recrystallized
from a mixture of dried ethanol and diethyl ether.
PTAB was treated in a dry box purged with nitrogen
gas, because this material was highly hygroscopic.
The infrared spectra were measured for DPA in an
argon matrix and the liquid state and for PTAB in the
solid state. The spectra in the liquid state and the solid
state (Nujol mull) were recorded on a Nicolet Impact
400 infrared spectrometer using a DTGS detector by
4. Results and discussion
4.1. Energies of conformers
Table 1 gives the relative energies and the skeletal
torsion angles for five possible conformers of DPA
and two possible conformers of PTA ion, along with

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K. Ohno et al./Journal of Molecular Structure 435 (1997) 219–228
(C)CH3 group. The ab initio calculations in fact give a
nonbonded distance between this hydrogen atom and
˚
the nitrogen atom to be 2.76 A which is close to a sum
˚
of the van der Waals radii of these atoms, 2.7 A. These
results indicate that an appreciably strong attractive
interaction occurs between the methyl hydrogen atom
and the nitrogen atom separated by three carbon
atoms. This interaction may therefore be called the
…
intramolecular 1,4-C–H N hydrogen bonding. It is
also noted that the length of the C–H bond involved in
this interaction is shorter than the length of the normal
C–H bonds; namely, the calculated length for the
interacting C–H bond in the GЈG conformer is
˚
1.0833 A, while the lengths for other C–H bonds in
˚
the same methyl group are 1.0861 and 1.0868 A.
This is the same structural tendency as found for
…
…
O
the 1,4-CH O interaction [14] and the 1,5-CH
interaction [29].
The difference in the conformational stability
between the GT and TT conformers of DPA, both of
which have the trans NCH2–CH2CH3 bond, is due to
the different number of the gauche conformation for
the CH3N–CH2CH2 bond involved in the respective
conformers. The difference in the conformational
stability between the GG and TG conformers is also
explained by the different number of the gauche
conformation for the CH3N–CH2CH2 bond.
Fig. 1. Structures of five possible conformers of N,N-dimethyl-
propylamine (DPA) and two possible conformers of propyl-
trimethylammonium (PTA) ion. The dashed arrow indicates an
…
intramolecular C–H N hydrogen bond.
the number of the gauche conformation for the
CH3N–CH2CH2 bond and other important conforma-
tions involved. The structures of these conformers are
shown in Fig. 1. For DPA, the three conformers with
gaucheЈ–gauche (GЈG), gauche–trans (GT) and
trans–trans (TT) around the LN–CH2–CH2CH3
bonds, where L denotes the lone pair, are more stable
than the gauche–gauche (GG) and trans–gauche (TG)
conformers by roughly 3 kcal mol−1. For PTA, the
trans (T) conformer is more stable than the gauche
(G) conformer by 3.4 kcal mol−1. The much higher
energies for the GG and TG conformers of DPA and
the G conformer of PTA are obviously associated with
strong steric repulsions between the CH3(N) and
(C)CH3 groups, which are close to each other owing
to the gaucheЈ–gauche conformation for the CH3N–
CH2–CH2CH3 bonds.
It is noteworthy that the GЈG form is the most stable
conformer of DPA. The origin of this high stability is
explained by peculiar conformational dispositions,
involved only in this form, of the lone pair on the
nitrogen atom and the (C)CH3 group; namely, they
are in the gaucheЈ–gauche conformation. This
conformational disposition leads to proximity of the
nitrogen atom and one of the hydrogen atoms of the
4.2. Spectral analysis and conformation of
N,N-dimethylpropylamine (DPA)
The Raman spectra of DPA in the liquid and glassy
states and in aqueous solution are shown in Fig. 2. The
observed wavenumbers and approximate relative
intensities are listed in Table 2, where the calculated
wavenumbers and intensities for the most stable three
conformers, GЈG, GT and TT, are also given. The
assignments of the observed bands to the particular
conformers are almost straightforward owing to the
accurate results of the calculations. The crystalline
solid state of this compound was not obtained in
spite of several attempts by different methods, pos-
sibly because of the crystallization competition
between the two equally stable conformers, GЈG and
GT, in the low-temperature liquid.
The spectra in Fig. 2 indicate that some of the bands
change their intensities when going from the liquid
state to the low-temperature glassy state. The bands