Theoretical, structural, vibrational, NMR, and thermal evidence of the inter- versus intramolecular hydrogen bonding in oxamides and thiooxamides

Article abstract of DOI:10.1021/jp0401776

This contribution describes the study of hydrogen bonding in secondary oxamides, monothiooxamides, and dithiooxamides by ab initio calculations, X-ray diffractions, NMR spectra, thermal analysis, and variable-temperature infrared and Raman spectroscopy. The results can all be interpreted as a function of the change in the strength and the nature of the hydrogen bonding by substituting oxygen for sulfur in the series CH₃-HNCOCONHCH₃, CH ₃HNCSCONHCH₃, CH₃HNCSCSNHCH₃ and by changing the steric influence of the alkyl group in a series of oxamides (RHNCOCNHR; R = CH₃, C₂H₅, iC₃H ₇, tC₄₉).

Full text of DOI:10.1021/jp0401776

J. Phys. Chem. A 2004, 108, 5175-5182
5175
Theoretical, Structural, Vibrational, NMR, and Thermal Evidence of the Inter- versus
Intramolecular Hydrogen Bonding in Oxamides and Thiooxamides
,
†
‡
†
§
†
H. O. Desseyn,* S. P. Perlepes, K. Clou, N. Blaton, B. J. Van der Veken,
†
|
R. Dommisse, and P. E. Hansen
Department of Chemistry, UniVersity of Antwerp, RUCA, Groenenborgerlaan 171, 2020 Antwerp, Belgium,
Laboratory of Inorganic Chemistry, Department of Chemistry, UniVersity of Patras, Rio-Patra, Greece,
Laboratory of Analytical Chemistry & Medicinal Physicochemistry, UniVersity of LeuVen, LeuVen, Belgium,
and Department of LiVe Science and Chemistry, Roskilde UniVersity, P.O. Box 260,
DK-4000 Roskilde, Denmark
ReceiVed: February 26, 2004
This contribution describes the study of hydrogen bonding in secondary oxamides, monothiooxamides, and
dithiooxamides by ab initio calculations, X-ray diffractions, NMR spectra, thermal analysis, and variable-
temperature infrared and Raman spectroscopy. The results can all be interpreted as a function of the change
in the strength and the nature of the hydrogen bonding by substituting oxygen for sulfur in the series CH
HNCOCONHCH , CH HNCSCONHCH , CH HNCSCSNHCH and by changing the steric influence of the
alkyl group in a series of oxamides (RHNCOCNHR; R ) CH , C , iC , tC ).
3
-
3
3
3
3
3
3
2
H
5
H
3 7
4 9
H
Introduction
splitter, and a liquid-nitrogen-cooled MCT detector. Spectra at
-
1
a resolution of 1 cm were recorded using interferograms
averaged over 100 scans, Happ Geuzel apodized, and Fourier
transformed by applying a zero filling factor of 2.
In this work, we studied the change in the character of the
hydrogen bonding in two series of original compounds. The
hydrogen bonding has been studied using different techniques.
X-ray structures and ab initio calculations give direct information
because the most reliable criterion is based on the van der Waals
Solid-state low-temperature infrared spectra were obtained
using a cryostat consisting of a sample holder fixed to the bottom
of a liquid-nitrogen container suspended in a vacuum shroud.
The cryostat is optically coupled to the spectrometer using KBr
windows. All solid-state samples were studied in KBr pellets.
Fourier transform Raman spectra were recorded on a Bruker
IFS 66v interferometer equipped with an FT Raman FRA 106
module. The compounds were excited by the 1064-nm line of
a Nd:YAG laser operating at 200 mW. For each spectrum, 1000
scans were recorded and averaged. Low-temperature Raman
spectra were recorded in a Miller-Harney cell using a SPEX
1
2
radii and also the angle involved in the hydrogen bonding.
Vibrational spectroscopy has always been the main experimental
method in the study of hydrogen bonding, and we obtained very
valuable information from the temperature-dependent solid-state
3
-5
spectra,
especially when solubility problems were encoun-
tered. Concentration- and temperature-dependent NMR spectro-
scopy6-23
and especially the enthalpies confirm the trend
indicated by X-ray and vibrational spectroscopy between inter-
and intramolecular character, as indicated by structural and
vibrational spectroscopy.
The comparison of these different techniques clearly shows
that infrared and Raman spectroscopy are the best techniques
for these typical kinds of studies; the other techniques used in
this work also give important information throughout the whole
study.
The compounds under investigation are original oxamides,
thiooxamides, and dithiooxamides. These molecules form an
important group of ligands in coordination chemistry.
remarkable characteristic of these ligands is their thermal and
chemical stability and especially their dissociation constants.
1
403 double monochromator. The compounds were excited by
+
a Spectra Physics model 2020 Ar laser. The spectra were
recorded with a spectral slit width of 4 cm .
-
1
Enthalpies were measured on a TA instrument DSC 2920
equipped with an RCS cooling system and on an SDT 2960
simultaneous DSC-TGA instrument. The experiments were
-1
performed under a dry nitrogen atmosphere (50 mL min ), and
-
1
the heating rate was 1 K min . The thioamides under
investigation are known to be good complexing agents with very
soft acids. Because zero-valent metals in the standard DSC all
are very soft acids, they tend to form complexes, especially with
the thioamides, resulting in a deterioration of the cell, causing
considerable changes in the cell constant. Therefore, the recently
developed DSC-TGA upgrade for the SDT made of corrosion-
resistant material was used in the present measurements.
The NMR spectra were recorded on a Varian 400-MHz
spectrometer; the chemical shifts are given in ppm. DMSO-d6
was used as a solvent, and TMS was used as an internal
standard.
2
4
A
2
5
The very important role of hydrogen bonding in these various
characteristics has been extensively studied.5,26-32
Experimental Section
Infrared spectra were recorded on a Bruker IFS 113v Fourier
transform spectrometer using a globar source, a Ge/KBr beam
*
Corresponding author. E-mail: jozef.janssens@ua.ac.be.
University of Antwerp.
University of Patras.
University of Leuven.
Roskilde University.
†
X-ray Crystallography. Crystals were mounted on a glass
fiber using epoxy glue. A preliminary examination and data
collection were performed on a computer-controlled four-circle
‡
§
|
1
0.1021/jp0401776 CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/21/2004

5176 J. Phys. Chem. A, Vol. 108, No. 24, 2004
Desseyn et al.
TABLE 1: Crystallographic Data for Compounds DMMTO,
DMDTO, DEO, DiPO, and DtBO
RHNCSCONHR derivatives have been prepared according
to the following reactions:
parameter
DMMTO DMDTO
DEO
DiPO
DtBO
P2O
5
chemical formula C
4
H
8
N
2
OS C H N S C H N O C H N O C H N O
4 8 2 2 6 12 2 2 8 16 2 2 10 20 2 2
H NCOCOOC H (Fluka 619083)
8 NCCOOC H5
fw
132.18
P1
148.24
P2 /c
144.18
P2 /n
172.23
P2 /c
200.28
Pbcn
2
2
5
Tv
2
space group
a, Å
1
1
1
H2S
6.234 (2) 17.141 (5) 5.077 (1) 5.154 (1) 9.989 (3)
NCCOOC H
8 H NCSCOOC H
5
b, Å
c, Å
6.700 (1) 4.194 (1) 10.016 (1) 9.833 (1) 11.455 (3)
8.105 (2) 10.366 (6) 8.226 (1) 10.522 (1) 10.300 (4)
89.18 (1)
80.35 (3) 107.44 (4) 97.80 (1) 101.89 (1)
80.05 (1)
2
5
2
2
toluene
R, deg
â, deg
γ, deg
V, Å3
Z
reflux
H NCSCOOC H + 2.2RNH toluene8 RHNCOCSNHR
2
2
5
2
328.7 (1) 710.9 (5) 414.5 (1) 521.9 (1) 1178.6 (6)
2
The pale-yellow compounds were purified by recrystallization
from ethanol.
4
2
2
8
T, K
293 (2)
1.54178
1.336
3.642
0.0413
0.1213
293 (2)
0.71073 1.54178
1.385
0.649
0.0347
0.0888
293 (2)
293 (2)
1.54184
1.096
0.645
0.0761
0.2076
293 (2)
1.54178
1.129
0.633
0.0443
0.1152
λ, Å
The single crystals for the X-ray diffraction experiments were
prepared by the double-layer method (i.e., by adding hexane to
a solution of the compound in CH2Cl2).
D
calcd
1.155
0.724
0.0591
0.1618
-1
µ, mm
a
R1
wR2b
Computational Details. Density functional theory calcula-
tions were performed using Gaussian 98. For all calculations,
a
b
2
2 2
R1
)
Σ. (|Fo|-|Fc|)/Σ(|Fo|). wR2
)
{Σ[w(Fo - Fc ) ]/
37
2
2
1/2
Σ[w(Fo ) ]}
3
8
Becke’s three-parameter exchange functional was used in
combination with the Lee-Yang-Parr correlation functional,39
and the B3LYP/6 31++G** basis set was used. To reduce the
errors arising from the numerical integration, the “fine-grid”
option, corresponding to roughly 7000 grid points per atom,
was used for all calculations. To obtain information about
multimers (n ) 2, 3, 4, 5, and 6) the equilibrium geometry was
calculated without structural restrictions. For all equilibrium
geometries, the vibrational frequencies and the intensities were
calculated using harmonic force fields.
diffractometer with graphite monochromator (Siemens P4 using
Cu KR or Stoe Stadi4 using Mo KR radiation) at room
temperature.
Omega scans for several intense reflections were used as a
criterion for acceptable crystal quality. Three standard reflections
monitored every 100 reflections indicated crystal stability. A
3
3
semiempirical absorption correction was employed. The
structures were solved by direct methods.34
Further crystallographic details are given briefly as follows.
Compound DMMTO: 2θ(max) ) 137.6° (Cu KR radiation),
reflections collected/unique 1497/1169 (Rint ) 0.0567), 76
Results and Discussion
-
3
parameters refined, (∆F)max/(∆F)min ) 0.204/-0.202e Å .
Compound DMDTO: 2θ(max) ) 50.0° (Mo KR radiation),
reflections collected/unique 4554/1245 (Rint ) 0.0482), 76
1. X-ray Study. The most reliable criterion of hydrogen bond
formation is that based on the van der Waals radii. The van der
Waals radii of the atoms that are possibly engaged are 1.55
-
3
parameters refined, (∆F)max/(∆F)min ) 0.220/-0.188 e Å .
Compound DEO: 2θ(max) ) 137.8° (Cu KR radiation), reflections
collected/unique 1116/724 (Rint ) 0.0328), 56 parameters
40
(
N), 1.52 (O), and 1.80 Å (S). Thus, for N‚‚‚O and N‚‚‚S
distances shorter than 3.07 and 3.35 Å, respectively, the presence
of a hydrogen bond must be considered. We can also consider
H‚‚‚A distances with the van der Waals radius of the H atom
-
3
refined, (∆F)max/(∆F)min ) 0.184/-0.153e Å . Compound
DIPO: 2θ(max) ) 137.9° (Cu KR radiation), reflections collected/
unique 1402/918 (Rint ) 0.0280), 61 parameters refined, (∆F)max/
4
0
ranging from 1.00 to 1.20 Å, resulting in hydrogen bonds for
H‚‚‚O and H‚‚‚S distances between 2.52 and 2.72 Å and 2.80
and 3.00 Å, respectively.
-
3
(
1
1
)
∆F)min ) 0.298/-0.241e Å . Compound DTBO: 2θ(max) )
38.5° (Cu KR radiation), reflections collected/unique 1414/
028 (Rint ) 0.0430), 71 parameters refined, (∆F)max/(∆F)min
Because of the uncertainties in the hydrogen positions, we
-
3
will analyze the hydrogen bonds in terms of the N‚‚‚O and N‚
0.186/-0.189e Å .
‚
‚S inter- and intramolecular distances, although the H‚‚‚O and
For all structures, all non-hydrogen atoms were refined
H‚‚‚S distances shown in the Tables generally confirm the
results obtained from the N‚‚‚O and N‚‚‚S distances.
anisotropically. Most hydrogen atoms were geometrically ideal-
ized and allowed to ride on their parent atoms (O-H ) 0.82,
C-H ) 0.96, and N-H ) 0.86 Å). The isotropic displacement
Table 2 compares relevant experimental and calculated
(monomer) distances and angles involved in the hydrogen
parameters of all hydrogen atoms were assigned to be 1.3 times
those of their parent atoms.35
bonding in DMO, DMMTO, and DMDTO.
The crystal structure of CH3HNCOCONHCH3 has been
described. For the other compounds CH3HNCOCSNHCH3
DMMTO), CH3HNCSCSNHCH3 (DMDTO), C2H5HNCOC-
ONHC2H5 (DEO), iC3H7HNCOCONHiC3H7 (DiPO), and tC4H9-
HNCOCONHtC4H9 (DtBO), we gathered the crystallographic
data in Table 1.
DMO appears in the trans-planar configuration, exhibiting
the typical intermolecular peptide hydrogen bonding as given
in Figure 1. Both the inter- and intramolecular N‚‚‚O distances
(2.720 and 2.860 Å) are shorter than the van der Waals radius
(3.07 Å), clearly indicating the bifurcated hydrogen bonding in
these molecules.
3
6
(
Synthesis of the Compounds. RHNCOCONHR derivatives
were synthesized by reacting CH3OOCCOOCH3 with 2.2 equiv
of RNH2 in ethanol under reflux for 1/2 h. The colorless
compounds were purified by recrystallization from ethanol.
RHNCSCSNHR derivatives were synthesized by reacting
dithiooxamide (H2NCSCSNH2) with 2.2 equiv of RNH2 in
ethanol under reflux for 2 h. The colored compounds were
purified by recrystallization from ethanol.
DMDTO molecules also appear in the trans-planar configu-
ration as given in Figure 2. The molecule exhibits two slightly
different CN and CS distances per molecule; this phenomenon
has already been described in the structural study of the
hydroxyethyl derivative. The inter- and intramolecular N‚‚‚S
distances (average) of 2.943 and 3.484 Å are very different,
and the intermolecular N‚‚‚S distance is even considerably
greater than the sum of the van der Waals radii (3.35 Å), leading
4
1

Hydrogen Bonding in Oxamides and Thiooxamides
J. Phys. Chem. A, Vol. 108, No. 24, 2004 5177
Figure 2. Packing of the molecules in the crystalline phase of DMDTO.
Figure 1. (A) Infinite chains of hydrogen-bonded oxamide units in
the crystal structures of DMO (R ) CH
R ) iC ). (B) Packing of the molecules in the crystal structure of
DtBO.
3 2 5
), DEO (R ) C H ), and DiPO
(
3 7
H
TABLE 2: Inter- and Intramolecular Distances (Å) and
Angles in DMO, DMMTO, and DMDTO Involved in
Hydrogen Bonding
Figure 3. Hydrogen bonding in the crystalline phase of DMMTO.
N‚‚‚O and N‚‚‚S distances, we can conclude that bifurcated
DMO
DMMTO
DMDTO
CO
calcd
exptl
calcd
exptl
calcd
exptl
calcd
exptl
calcd
exptl
calcd
exptl
exptl
exptl
calcd
exptl
calcd
exptl
calcd
exptl
calcd
exptl
calcd
exptl
calcd
exptl
exptl
calcd
exptl
exptl
calcd
exptl
1.230
1.226
1.233
1.218
1.670
1.663
1.343
1.317
1.330
1.306
2.602
2.627
3.059
3.020
2.860
3.433
125.63
124.42
125.82
125.19
119.49
119.66
124.55
122.92
111.18
114.22
114.61
114.72
2.078
2.214
2.054
2.802
2.493
2.559
CS
1.679
1.673/1.665
CNamide
CNthioamide
O‚‚‚Nintra
S‚‚‚Nintra
1.347
1.328
1.329
1.458/1.309
exist.
Although the intermolecular S‚‚‚N distance (3.433 Å) is
longer than the sum of the van der Waals radii, it is shorter
than the S‚‚‚N intermolecular distance observed for DMDTO
(3.388 Å). For DMMTO, we can conclude that the hydrogen
bonding for the amide hydrogen obtains more intramolecular
character compared to DMO and that the thioamide hydrogen
shows more intermolecular character compared to DMDTO.
The influence of the cooperative effect in hydrogen-bonded
oligomers of DMO on the strength of the hydrogen bonding
and the effect on the CO and CN distances was studied using
DFT calculations on the monomer (x ) 1), trimer (x ) 3), and
pentamer (x ) 5). The main results have been collected in Table
2.695
2.720
2.971
2.945/2.942
O‚‚‚Ninter
S‚‚‚Ninter
2.860
3.479/3.488
∠
∠
∠
∠
∠
∠
NCO
NCS
CCO
CCS
NHO
NHS
126.67
125.26
124.11
124.52/124.52
121.76
121.19
122.48
121.73/121.13
108.22
112.88
3
. It can be seen that the pentamer exhibits longer CO and NH
but shorter CN bond lengths compared with those of the
monomer, suggesting stronger intermolecular hydrogen bonds
with the increasing number of monomers in the chain. This
increasing intermolecular strenght is also demonstrated by the
average hydrogen bond energy defined as
117.65
117.55/117.96
H‚‚‚Ointer
H‚‚‚Ointra
2.163
2.213
2.309
H‚‚‚Sinter
H‚‚‚Sintra
>3.50
2.355
2.444
E(DMO)x - xEDMO
(x ) number of molecules)
x - 1
to the conclusion that in DMDTO we can mainly consider strong
intramolecular hydrogen bonding, although the packing of the
molecules as given in Figure 2 suggests some intermolecular
interaction.
-1
This energy increases from 22.4 kJ mol in the dimer to 23.0
-1
kJ mol in the pentamer. For N-methylacetamide, exhibiting
only intermolecular hydrogen bonds, this energy changes from
-
1
4
DMMTO molecules also exhibit a planar-trans configuration
31.8 to approximate 39 kJ mol for the pentamer. Thus, the
intermolecular interactions in DMO are about 30% weaker
(
Figure 3). From the inter- and intramolecular alternating

5178 J. Phys. Chem. A, Vol. 108, No. 24, 2004
Desseyn et al.
TABLE 3: Cooperative Effect in DMOa
TABLE 5: Comparison of Some Fundamentals Influenced
a
by Hydrogen Bonding in DMO, DMMTO, and DMDTO
solid state
monomer trimerd pentamerd 25 °C -196 °C
DMMTO
DMO
O
S
DMDTO
N-Hb
νNH
CNc
1.014
3400
1.347
1518
1215
1.230
1691
1.019
3335
1.340
1537
1254
1.235
1684
1.971
2.299
149.00
103.17
1.019
3328
1.337
1538
1259
1.238
1681
1.956
2.379
150.45
101.91
3312
1558
3299
1560
νNH exptl 25 °C
3312 3303 3247
13
765
20
3180
0
712
0
1538
1538
b
∆ν
4
3
νCN
amide II
amide III
πNH
25 °C
∆ν
25 °C
817
4
681
7
πNH
25 °C
b
COb
1.293
1660
1.295
1658
amide I
amide II
amide III
1660 1677 1535 νCN
25 °C
-190 °C
νCO
amide I
-190 °C 1657 1677 1537
25 °C 1533 1560
-190 °C 1537 1560
25 °C 1237 1240
O‚‚‚Hinterc
O‚‚‚Hintrab
2.200
b
∠
∠
OHNinter
OHNintrac
108.22
-190 °C 1240 1240
a
-1
Distances (Å); angles and frequencies (cm ) (See footnote d.)
Highest values in the series. c Lowest value in the series. d The
νNHdil. CH Cl
νNHsolid (25 °C)
∆ν
2
2
b
DMO
3412
3341
3300 sh
3192
3312
3303
3247
3180
100
38
(53)
12
frequency of the relatively most intense band from the series is given.
DMMTO (O)
S)
DMDTO
(
TABLE 4: Distances and Angles in DMO, DEO, DiPO, and
DtBO Involved in Hydrogen Bondinga
a
IR band in cm-1. b ∆ν ) ν(298 K) - ν(77 K).
DMO42
compound
DEO
DiPO
DtBO
CO
CN
exptl
exptl
exptl
exptl
exptl
exptl
calcd
exptl
1.226
1.328
2.720
2.860
2.309
2.163
3400
3412
1.226
1.317
2.704
2.893
2.286
2.100
3400
3399
1.212
1.320
2.699
2.997
2.403
2.111
3384
3387
1.229
1.330
2.683
3.568
2.250
2.692
3374
3377
2
. Vibrational Spectra. Table 5 gives the νNH fundamentals
in the solid state at different temperatures and in dilute CH2Cl2
solutions and some other significant fundamentals in the solid
state of DMO, DMMTO, and DMDTO. The νNH in the solid
state of secondary amides and thioamides depends on the acidity
O‚‚‚Nintra
O‚‚‚Ninter
H‚‚‚Ointra
H‚‚‚Ointer
νNH
(
thioamides are more acidic than amides and the νNH modes
νNHdil.CH Cl
2
2
appear at lower frequencies), the mass of the alkyl substituent,
and the geometry of the molecule, so from the position of the
νNH in the solid state, no conclusions can be made. However,
the ∆ν values (i.e., the difference between the νNH in the solid
state and the νNH of the compound in dilute CH2Cl2) give a
direct indication of the strength of the intermolecular hydrogen
bonding. From these ∆ν values, given in Table 5, we can learn
that DMO exhibits intermolecular hydrogen bonds and that
intermolecular hydrogen bonding in DMDTO is nonexistent
a
Distances in Å; ν in cm-1
.
compared with those of N-methylacetamide. This difference
must be assigned to the considerable intramolecular interaction
in DMO.
Table 3 also lists the vibrational frequencies for some
characteristic amide fundamentals, calculated for the monomer
and multimers, and the experimental data of the solid-state
spectra at different temperatures.
These frequency shifts on passing from the monomer (gas
phase) to the multimers to the solid state at room temperature
x ) ∞) and the shift observed at lower temperature can be
well explained by considering stronger intermolecular hydrogen
bonding in this series, which is due to the cooperative effect
and the fact that the change in temperature in the solid state
causes a change in the intermolecular distances and consequently
a change in the donor-acceptor strength, resulting in stronger
intermolecular interactions at lower temperatures.4
Table 4 compares relevant distances concerning hydrogen
bonding for oxamides exhibiting different CH3 groups on the
R carbon of the alkyl group.
The CO and CN bond lengths are all comparable; in both
cases (inter- and intramolecular hydrogen bonds), we expect
longer CO and shorter CN bond lengths due to the resonance-
assisted intra- and intermolecular hydrogen bonding. From the
changes in the inter- and intramolecular N‚‚‚O distances, we
can clearly conclude that the intramolecular character increases
with the steric hindrance, which is caused by the number of
CH3 groups on the R carbon of the alkyl groups. Furthermore,
it is very clear that the hydrogen bonding in the tC4H9 derivative
is practically purely intramolecular because the intermolecular
N‚‚‚O distance is greater than 3.568 Å. The fact that the tC4H9
derivative does not exhibit the typical linear peptide structure
is caused by this pure intramolecular character in the hydrogen
bonding (Figure 1B).
-
1
because the small ∆ν value (12 cm ) must be ascribed to the
44
interactions of the molecules in the solid state. The amide νNH
mode shows less intermolecular association in DMMTO, but
this intermolecular character becomes more pronounced for the
thioamide νNH mode in DMMTO compared with that in
DMDTO.
(
This increased intramolecular character of the amide and the
increased intermolecular character of the thioamide group in
DMMTO are also confirmed in the solid-state spectra at different
2,43
-1
temperatures. νNH of DMO shifts about 13 cm to lower
-1
frequency, and πNH shifts 20 cm to higher frequency, which
is indicative of intermolecular hydrogen bonding. The zero shift
of the νNH and πNH in DMDTO clearly indicate pure
intramolecular hydrogen bonding. The shift observed for DM-
MTO again indicates more intramolecular character for the
amide and less intramolecular character for the thioamide
hydrogen bonding.
The shift to lower frequency of the amide I band (mainly
νCO) and the shift to higher frequency of the amide II and amide
III band (δNH + νCN) by lower temperature confirm the
intermolecular character in DMO. The zero shift of the νCN in
the thioamide group indicates pure intramolecular character in
DMDTO.
Again for DMMTO we observe smaller frequency shifts for
the amide fundamentals and a slight shift of the νCN of the
thioamide function, confirming more intramolecular character
for the amide group and more intermolecular character for the
thioamide group interaction in DMMTO compared with that in
DMO and DMDTO, respectively.
Table 4 also gives the theoretical νNH in the gas phase and
the νNH frequencies in very dilute CH2Cl2. In both series, we
observe a decreasing νNH frequency, indicating the increasing
intramolecular character.

Hydrogen Bonding in Oxamides and Thiooxamides
J. Phys. Chem. A, Vol. 108, No. 24, 2004 5179
TABLE 7: NMR Dataa
a
δ/δT (ppm/K), t1/2 (min), and ∆δ/∆C (ppm/M). b Not soluble.
∆
Figure 4. νNH modes of different butyloxamides in dilute CH
TABLE 6: Comparison of νNH Modes in the Solid State
2 2
Cl .
-
1
and in Dilute CH
2
Cl
2
Solutions (cm ) of Some Oxamides
-
1
(
cm )
νNHsolid
compound
νNHdil. CH Cl2 (25 °C) ∆ν
Figure 5. δ as a function of Τ(K) for the amide protons in DMO.
2
MA
methylacetamide
(CH HNCOCONHCH
(C HNCOCONHC
3461
3412
3399
3387
3377
3397
3404
3384
DMO
DEO
DiPO
DtBO
DBO
DiBO
DsBO
3
3
character compared with that in DMO, and the thioamide proton
has stronger intermolecular character compared with that in
DMDTO).
Concentration studies from 0.005 to 1 M (0.15 M for DMO)
were carried out in CDCl3 to minimize solvent interactions. The
results were expressed as a ∆δ/∆C value and are also listed in
Table 7.
A comparison of the ∆δ/∆C values of the amide proton in
DMO and DMMTO shows that the former one has more
pronounced intermolecular interaction. The extremely low value
of ∆δ/∆C for the thioamide hydrogen in DMDTO, however,
results from the strong intramolecular interaction in this
compound.
2
H
5
2
)
7
(iC
(tC
3
H
H
7
HNCOCONHiC
HNCOCONHtC
)
)
4
9
9
(C
(iC
(sC
4
H
9
HNCOCONHC
HNCOCONHiC
HNCOCONHsC
4 9
H )
H
4 9
9
)
4
H
9
H
9
)
I.R. νNH
Raman
-196 °C
2
5 °C
-120 °C
∆ν
25 °C
DMO
DEO
DiPO
DtBO
MA
3312
3304
3298
3338
3304
3299
3289
3291
3336
3284
3319
3303
3299
3334
3300
Temperature coefficients (∆δ/∆T) have been used routinely
in amides and thioamides to gauge intramolecular hydrogen
8
,11,17,19
Table 6 gives the ∆ν values in the solid-state infrared and
Raman spectra (Bu and Ag) and the ∆ν differences between
the solid-state spectra at 21 °C and the νNH in very dilute CH2-
Cl2 for the oxamides. For methylacetamide (MA), exhibiting
only intermolecular hydrogen bonds, we observed considerable
bonding.
The ∆δ/∆T values are linear within the
experimental error ((0.03 ppm), as can be seen from Figure 5
for the amide protons in DMO.
-
3
∆δ/∆T values in DMSO are situated between -4 × 10
and -12 × 10 ppm/K for intermolecular systems and between
-1 × 10 and 2 × 10 for strong intramolecular hydrogen
bonding.
The spectra were recorded at 30, 60, 90, 120 and 150 °C
-3
-
3
-3
∆ν values from the solid-state spectra at different temperatures
1
4,32,45-47
as well as from the difference between the νNH in the solid
state and the νNH in dilute CH2Cl2. Consequently, these shifts
are a measure of the intermolecular forces, and they decrease
with increasing steric hindrance.
Because the νNH of secondary amides also depends on the
mass of the alkyl group, we studied the four butyl groups,
exhibiting the same mass but different in their steric hindrance.
Figure 4 nicely shows the influence of the steric effect on the
free νNH modes.
-
3
(-3 × 10 ppm/K is given in Table 7 as 3 ppm/K).
The temperature coefficients clearly show intramolecular
hydrogen bonding in DMDTO and intermolecular
3
. NMR Spectra. DMO, DMMTO, and DMDTO were
interactions in DMO.
dissolved in dioxane-d8 containing 1.8% D2O, and the exchange
rate was determined by integrating the N-H signal intensity.
The observed half-life times are given in Table 7.
The hydrogen atoms of the thioamide groups appear to
exchange much faster because of the more acidic character.
These results also confirm the structural and vibrational results
All three ∆δ/∆T, t , and ∆δ/∆C values indicate more
1
/2
intramolecular character for the amide group and less intramo-
lecular character for the thioamide group in DMMTO than in
DMO and DMDTO.
By comparing the oxamides, we can distinguish between only
DtBO and DMO, DEO and DiPO. The intramolecular associa-
tion observed from the structural and vibrational data is too weak
(i.e., the amide proton in DMMTO has stronger intramolecular

5180 J. Phys. Chem. A, Vol. 108, No. 24, 2004
Desseyn et al.
TABLE 8: DSC Values for Some Amides and Thiomides
no. of
intermolecular
H bonds
product
∆totalH*
R
(
(
(
(
(
CH3)2NCOCON(CH3)2
CH3)2NCSCON(CH3)2
CH3)2NCSCSN(CH3)2
C2H5)2NCOCON(C2H5)2
C3H7)2COCON(C3H7)2
I
II
70
0
0
0
0
0
4
2
2
2
2
2
2
2
0
2
4
4
4
75
82
80
88
109
83
III
IV
V
VI
VII
VIII 94
IX
X
XI
XII
XIII 97
XIV 79
H2NCOCONH2
CH3NHCOCONHCH3
C2H5NHCOCONHC2H5
C3H7NHCOCONHC3H7
C4H9NHCOCONHC4H9
iC4H9NHCOCONHiC4H9
sC4H9NHCOCONHsC4H9
tC4H9NHCOCONHtC4H9
CH3HNCSCSNHCH3
CH3HNCOCSNHCH3
0
1
1
1
1
2
3
105
123
118
112
XV
82
HOC2H4HNCOCONHC2H4OH XVI 146 (cfr. propyl)
HOC3H6HNCOCONHC3H6OH XVII 152
HOC4H8HNCOCONHC4H8OH XVIII 166
TABLE 9: DSC and TGA Data for N,N′-Dipropyloxamide
at Different Heating Rates
â
∆
totalH*
kJ/mol
T(dm/dT)max
°C
-
1
°
C min
residue %
0
0
1
2
0
5
0
.25
.5
91
92
92
91
92
95
98
139
148
146
165
192
200
203
3.8
4.6
6.8
Figure 6. Solid f gas-phase transitions for DPO at different heating
rates.
8.4
1
1
2
13.7
15.3
18.8
to resist the strong interaction with the solvent (DMSO). In
DtBO, however, the strong intramolecular character is con-
firmed.
4
. DSC Measurements. Table 8 contains the DSC data
(∆totalH*) for different amides and thioamides. These ∆totalH*
values all refer to the energy involved in the change from the
solid state to the gas phase. This ∆totalH* can also be the sum
of ∆meltH and ∆vapH and eventually some small contribution of
∆
decompH. This ∆decompH is acceptable because the contribution
is very small and also because we compare the results in a series
of compounds with only the number and strength of intermo-
lecular hydrogen bonds as variable components.
A typical example of the ∆totalH* is given in Table 9, and
the solid f gas phase transitions at different heating rates for
N,N′-dipropyloxamide are shown in Figure 6. At very low
heating rates (â values), the compound is completely sublimed
before the melting point; we further clearly observe the melting
and evaporation of the compound at higher â values. The residue
also given in Table 9 is a direct indication of the decomposition
of the compound. Therefore, ∆totalH* values at a low heating
Figure 7. Relations between ∆totalH* and the molecular mass in
oxamides and thiooxamides.
-
1
rate (1 °C min ) are used in Table 8.
It is very well known that in a series of comparable
compounds (comparable lattice energy) ∆totalH* increases with
increasing molecular mass. This is shown in the series (I, II,
III) where oxygen is replaced by sulfur (Figure 7A), in the series
By comparing VI, VII, and I, we see that despite the
increasing mass we observed a decrease in ∆totalH* values on
-
1
passing from H NCOCONH (109 kJ mol ) (four intermo-
2
2
-
1
lecular H bonds) to CH HNCOCONHCH (83 kJ mol ) (two
3
3
(
1
VII, VIII, IX, X) where the mass of the alkyl group (with R )
3 2 3 2
intermolecular H bonds) to (CH ) NCOCON(CH ) (70 kJ
mol ) (zero intermolecular H bonds) (Figure 7B).
-
1
) increases, and in the series (XVI, XVII, XVIII) with
increasing alkyl length in the hydroxyalkyloxamides.
By comparing the RHNCOCONHR-butyl derivatives (X, XI,
XII, XIII), we clearly can see that although these compounds
have the same mass the ∆totalH* decreases by increasing R. This
confirms the conclusion for the structural, vibrational, and NMR
data that by increasing R we observe a weaker intermolecular
and a stronger intramolecular interaction.
The number of intermolecular interactions is very important.
By comparing C3H7HNCOCONHC3H7 (mm 172) and HOC2H4-
HNCOCONHC2H4OH (mm 176), we find that the propyl
derivative exhibits only two and the hydroxyethyl derivative
exhibits only four intermolecular interactions (Figure 8).48
By comparing DMO (VII) ∆totalH* ) 83, DMMTO (XV)
∆totalH* ) 82, and DMDTO (XIV) ∆totalH* ) 79, we also can

Hydrogen Bonding in Oxamides and Thiooxamides
J. Phys. Chem. A, Vol. 108, No. 24, 2004 5181
Figure 8. Intermolecular hydrogen bonds in DPO and DHEO.
Figure 10. Solid-state spectra at different temperatures for the νNH
and νOH modes in D2HPO and D4HPO.
The νNH mode in D2HPO is not shifted at lower temperature,
indicating strong intramolecular interactions; these interactions
are confirmed by NH ∆δ/∆T ) 0.67 ppm/K. The NH modes
of D3HPO and D4HPO clearly show intermolecular associations.
The OH groups for the three compounds are all intermolecularly
associated, as can be learned from the vibrational and NMR
data.
For D2HPO we could very well calculate the ∆totalH*, whereas
for D3HPO and D4HPO, the compounds decompose before
subliming because of the four strong intermolecular interactions.
Figure 9. Geometry and hydrogen bonding in D2HPO.
TABLE 10: Comparison of Different
Hydroxyphenyloxamidesa
Conclusions
D
2
HPO
D
3
HPO
4
D HPO
Two interesting series of molecules with changing hydrogen
bond character have been studied by several techniques. All of
the results indicate the same trend, and the results can all be
regarded as substantial contributions in this study. It is also
remarkable that the enthalpy data as listed in Table 8 can be
interpreted only as a function of the nature of the hydrogen
bonding.
The article explains why DtBO and D2HPO do not exhibit
the typical linear peptide structure due to the strong intramo-
lecular hydrogen bonding in these molecules. The fact that
tC4H9HNCSCSNHtC4H9 cannot be prepared, either from DTO
solid-state IR spectra
νNH
νOH
25 °C
3356
3356
3307
3299
3295
3286
-
-
196 °C
25 °C
3311
3291
3340
3320
3394
3372
196 °C
∆
δ/∆T (ppm/K)
totalH* (kJ/mol)
T(dm/dT)max
no. of intermolecular
associations
OH
NH
5.87
0.67
205
301
2
4.50
5.13
4.60
4.97
∆
decomp. decomp.
335
4
360
4
a
ν (cm-1); T(°C).
+
tBNH2 or from refluxing tC4H9HNCOCONHtC4H9 with P2S5,
can be explained by assuming very strong intramolecular
N-H‚‚‚S hydrogen bonds, resulting in the formation of H2S
during the process.
state that despite the increasing mass the ∆totalH* is in large
part determined by the difference in the strength of the
intermolecular interactions.
Acknowledgment. We thank Greta Thijs for technical
assistance.
5
. Hydroxyphenyl Oxamides. F. J. Martinez-Martinez17
concluded from an NMR and X-ray diffraction study that in
N,N′-bis(2-hydroxyphenyl)oxamide (D2HPO) three-center in-
tramolecular hydrogen bonding, as given in Figure 9, is
comparable to that in DtBO, where the typical linear secondary
amide pattern is absent. The steric influence of both the
hydroxyphenyl group and the tertiary butyl group results in a
very short intramolecular N‚‚‚O distance (2.720 Å for DMO,
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