VIBRATIONAL SPECTROSCOPY ELSEVIER
Vibrational Spectroscopy 10 (1996) 169-175
Infrared spectra of cyclic and non-cyclic ureas in solution: structures and interactions Eugenia I. Harnagea, Paul W. Jagodzinski * Department of Chemistry, West Virginia University, P.O. Box 6045, Morgantown, WV2650, USA Received 11 March 1995
Abstract Infrared spectra of 1,3-dimethyl-2-imidazolidinone, 2-imidazolidinone, 1,1,3,3-tetramethylurea, 1,3-dimethylurea, and 1,l-dimethylurea have been collected in a variety of solvents. When the C=O stretching frequencies for the molecules in a common solvent were recorded, higher frequencies were found for the cyclic ureas as compared to the non-cyclic ureas. When data were analyzed within each class of urea derivatives, it was found that an increase in the number of methyl groups present on the nitrogen atoms contributed to a decrease in the C=O stretching frequency values. The frequencies of the C = 0 stretching modes have been correlated with the electron accepting ability of each solvent (represented by Gutmann electron acceptor numbers). The urea derivatives in the studied series exhibit similar behavior with respect to changing solvent environment despite the differences that exist among the ureas. These observations have been interpreted in terms of the structural characteristics of the ureas. Keywords: Infrared spectrometry; Ureas; Solvent effect; Carbonyl group; Electron acceptor number; Hydrogen bonding
1. Introduction It is well known that frequencies of many vibrational normal modes are dependent on the molecular environment. Infrared spectroscopy allows us to monitor solvent-solute interactions by following the solvent-dependent vibrational frequency shifts of the solute. There have been attempts to calculate solvent-induced shifts and to develop quantitatively accurate and physically meaningful explanations for observed solvent-induced frequency shifts of bond stretching modes of selected moieties [1-6]. Equations have been proposed in an attempt to model the
specific and non-specific factors that would best reflect the effect of the solvent on the solute. Some of these equations are as follows: K i r k w o o d - B a u e r - M a g a t (KBM) equation:
* Corresponding author. 0924-2031/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved
SSDI 0 9 2 4 - 2 0 3 1 ( 9 5 ) 0 0 0 4 6 - 1
v(vapor) -- v(soln)
c( 6 - 1)
( 2 e + 1)
Modified KBM equation: AV
c ' ( n 2 -- 1)
2n 2 + 1
Buckingham equation: Av e-- 1 n 2- 1 W = c1 + c 2 2 6 +----~ + ca 2n 2 + 1
E.L Harnagea, P.W. Jagodzinski / Vibrational Spectroscopy I0 (1996) 169-175
Linear Free E n e r g y Relationship ( L F E R ) equation: v = v ° + po"
W o h a r - S e e h r a - J a g o d z i n s k i equation: vS(hexane) - v s vS(hexane)
In these equations v ° is the observed vibrational frequency in the vapor phase, v s is the observed frequency in solution, e is the solvent dielectric constant, n is the solvent index of refraction, ~r is an empirical constant characteristic of the solute substituents, p is an empirical constant i n d e p e n d e n t of the solute substituents, A N is the solvent electron acceptor number, and c, c', c 1, c2, c3, a, and C ' are constants. Although n u m e r o u s equations have been developed, Laurence et al.  have pointed out that there is no u n i q u e scale for a n a l y z i n g s o l v e n t - s o l u t e interactions. Ureas have previously b e e n studied in a wide range of solvents and the A N approach has provided a reasonable explanation of s o l v e n t - s o l u t e interactions [5,7,8]. Therefore, we have chosen to interpret our data based on Eq. 5. The electron acceptor n u m b e r (AN), as proposed
by G u t m a n n , is a measure of solvent electrophilicity. A N represents the electron accepting capacity of the solvent and can be considered a molecular property . The A N values assigned for different solvents are based on N M R shifts (dependent on V a n der Waals interactions), solute-induced reaction fields (related to the dielectric constant of the solvent), and specific s o l v e n t - s o l u t e interactions. Acceptor n u m bers include factors that have been considered important for explaining solvent-induced frequency shifts and use an approach based on Lewis a c i d - b a s e concepts. In addition, these n u m b e r s can be considered as quantitative representations of the local association effects described b y B e l l a m y . The v ( C = O ) values were considered to be good indicators of the interactions of the urea derivatives with the solvent since the n o r m a l m o d e g i v i n g rise to the signal assigned as v ( C = O ) is c o m p o s e d of contributions from the entire carbamide skeleton [11,12]. Moreover, as it has been noted by Dobrowolski et al. , the C = O band of urea is more susceptible to intermolecular interactions than the N R 2 bands, where R = H, CH 3. Therefore, following the behavior of v ( C = O ) of substituted ureas in a chosen range of solvents can provide significant
Table 1 Variation of v(C=O) of DMEU, TMU, EU, 1,3-DMU, 1,1-DMU with solvent acceptor number (AN) Solvent
Acceptor number (AN) a
v(C=O) DMEU (cm- t)
v(C=O) TMU b (cm- 1)
v(C=O) EU (cm- l)
v(C=O) 1,3-DMU (cm- 1)
v(C=O) 1,1-DMU (cm- 1)
Neat liquid Hexane Tetrahydrofuran Benzene Carbon tetrachloride Hexamethylphosphoramide Acetone Nitrobenzene Benzonitrile Acetonitrile Dimethylsulfoxide Dichloromethane Chloroform Ethyl alcohol Methyl alcohol Water
1699.3 1716.7 1703.4 1703.3 1701.3 1700.7 1695.5 1697.5 1696.3 1695.5 1691.7 1691.2 1686.0 1687.2 1685.2 1655.9
1649.6 1668.3 1655.1 1652.1 1652.8 1648.9 1646.5 1644.4 1643.2 1641.9 1638.4 1637.4 1627.3 1630.1 1625.8 1583.6
1722.3 1710.8 1713.6 1712.9 1709.0 1712.9 1709.7 1708.8 1707.0 1681.2 1680.1 1652.9
1682.0 1680.0 1675.4 1678.0 1671.2 1667.3 1662.0 1642.3
1663.5 1662.5 1656.7 1653.8 1641.3
a Values taken from . b Values taken from .
0.00 8.00 8.20 8.60 10.6 12.5 14.8 15.5 18.9 19.3 20.4 23.1 37.1 41.3 54.8
E.I. Harnagea, P. W. Jagodzinski / Vibrational Spectroscopy 10 (1996) 169-175
insight into the understanding of solvent-solute interactions. While Dobrowolski et al. analyzed the solvent-solute interactions from the viewpoint of the solvent, we have analyzed it from the viewpoint of the solute. In 1988 a Raman investigation of the interactions of 1,1,3,3-tetramethylurea with various solvents was published . Other reports have analyzed the Raman spectra of methyl-substituted ureas in a few selected solvents [7,8,14-17]. In order to continue the investigation of solvent-solute interactions in ureas, a set of cyclic and non-cyclic urea derivatives was chosen for further study. These include: 1,3-dimethyl-2-imidazolidinone (dimethylethylene urea, DMEU), 2-imidazolidinone (ethylene urea, EU), 1,1,3,3-tetramethylurea (TMU), 1,3-dimethylurea (1,3-DMU), and 1,1-dimethylurea (1,1-DMU). This work allows us to discuss the effect of methyl groups as well as ring systems on v(C=O) and their effect on the interaction between the carbonyl C=O stretching mode of the ureas and the solvent environment. 2. Experimental 1,1,3,3-Tetrametylurea (TMU, 99% purity), 1,3dimethylurea (1,3-DMU, 99% purity), 1,1-dimethylurea (1,1-DMU, 99% purity), 1,3-dimethyl-2-imidazolidinone (DMEU, 98% purity), and 2-imidazolidinone (EU, 96% purity) were obtained from Aldrich (Milwaukee, WI) and used without further purification. Since they are hygroscopic substances they were stored in a desiccator over calcium sulfate. The common non-aqueous solvents necessary for this study were obtained from Fisher Scientific (Pittsburgh, PA) and were used without further purification. Solvents have been selected so that the spectral region corresponding to the expected v(C=O) frequencies was generally free of interfering signals. Infrared spectra were collected with a Mattson Instrument Cygnus 100 VF-IR spectrometer (Madison, WI) using thin films pressed between sodium chloride or zinc selenide windows. The number of interferograms collected was sample dependent and ranged from 8 interferograms for most samples to 64 interferograms for a few samples. Spectral resolution was set to 4.0 cm-1. We believe our frequencies are
accurate to within 2 cm -] for most peaks and to within 4 cm -1 for broader absorption bands. All measurements were performed at an average room temperature of 23°C. The small variations in room temperature had no observable effect on the position of the absorption bands. All spectra were collected 2-3 times with fresh samples of 0.1 M concentration and good reproducibility was observed.
3. Results The ureas that have been studied are shown in structural form in Fig. 1 and the vibrational data that have been collected are presented in Table 1. Plots of the observed v(C=O) values versus solvent AN have been constructed and are shown in Fig. 2. It can
II /c\ Me--N
I, ] ,3,3-Teb,amethylurea
~Me i ,3-Dimethylurea
!, I -DMU
Fig. 1. Structures of the ureas used in this study: 1,3-dimethyl-2-
imidazolidinone (dimethylethylene urea, DMEU); 2-imidazolidinone (ethyleneurea, EU); 1,3,3-tetramethylurea(TMU);1,3-dimethylurea(1,3-DMU); 1,1-dimethylurea(1,I-DMU).
E.L Harnagea, P. W. Jagodzinski / Vibrational Spectroscopy 10 (1996) 169-175
1740 1720 1700 C
1660. g 1640 -
1620 1600 1580 1560
1,3- DMU D: 1,1- DMU C:
E: TMU i
20 30 AccepterNumber
Fig. 2. Plot of v(C=O) (cm -1 ) for various ureas vs. solvent acceptor number. (A) EU; (B) DMEU; (C) 1,3-DMU; (D) 1,1-DMU; (E) TMU.
be seen that the values of v(C=O) are greater for the cyclic molecules than for the non-cyclic molecules in each of the solvents used in this study. In addition, within each of these groups, the molecules with the larger number of methyl groups have lower frequencies. It can be seen that v(C=O) decreases with increasing solvent AN for all of the ureas. Approximately linear and nearly parallel traces are observed for the ureas in the solvents with AN < 30. Hydrogen bonding perturbs the ureas in solvents with AN > 30 and different patterns are observed in this region of the plot. In the next section we will address the intramolecular dynamics that contribute to the ordering of v(C=O) and the intermolecular dynamics that result in the solvent dependence of v(C=O).
In order to understand the complex solvent-solute dynamics, we present the discussion from the viewpoint of the solute. First, we will discuss the effect of the ring and the methyl groups on the ordering of the v(C=O). Second, we will discuss the effect of the ring and the methyl groups of the ureas on the solvent-urea interactions. We considered EU and DMEU to be the cyclic
analogues of 1,3-DMU and TMU, respectively. As can be seen in Table 1 and Fig. 2, the values of v(C=O) for the cyclic compounds are larger than those for the non-cyclic compounds. In order to understand the factors responsible for the observed difference in frequency we have analyzed the effect of the ring on the electronic structure. Previous analysis of the vibrational and microwave spectra of EU, DMEU and their deuterated derivatives have been based on both C 2 (liquid and gas phases) and C2~ (liquid and solid phases) symmetry [17-19]. Semiempirical and ab initio calculations have shown that the C 2 structure should be more stable for both molecules . The five-membered 2-imidazolidinone ring is subject to energy strain due to torsional modes that promote a non-planar system, however, five membered rings have minimal strain since their angles (108 o) are nearly tetrahedral . These torsional modes are balanced by resonance stabilization effects that favor a planar structure which involves a high-energy conformation with eclipsed ring methylene groups. Based on this, we assumed non-planar structures in solution. Torsional strain and resonance effects favor non-planar structures with strong double-bond character for the C=O moiety. The values of v(C=O) for the cyclic molecules are therefore larger than the values for the
E.L Harnagea, P. W. Jagodzinski / Vibrational Spectroscopy 10 (1996) 169-175
non-cyclic molecules, where a more planar carbamide skeleton favors a weaker C = O bond. The effect of the solvents will be discussed later although a comparison of the traces for EU and DMEU in Fig. 2 is worthwhile at this time. In all cases where AN < 30, the values of v(C=O) are larger for EU than for DMEU. Therefore, EU has greater double-bond character in the C = O moiety than does DMEU. This is consistent with the results of 6-31G * * calculations and eliminates the inconsistency found when v ( C : O ) from infrared and Raman spectra were compared for these two molecules in different phases and in solution . It should be noted that a comparison of infrared C = O stretching frequencies with Raman C = O stretching frequencies of urea derivatives is problematic due to orientational and vibrational relaxation processes. When we consider the frequencies of the noncyclic ureas we observe a downward shift that follows the order 1,3-DMU > 1,1-DMU > TMU (see Table 1). It is well known that the electron donating ability of the amino moieties increases in the order H2N < CH3NH < (CH3)2N. This electron donating ability may be correlated in an inverse manner with the ionization potential of each moiety. We will use values of 9.0, 8.2, and 7.9 eV for the ionization potentials of H2N, CH3NH and (CH3)2N, respectively . If we assume that each of these moieties independently contributes to the total electron donating ability of each solute molecule, then we predict total electron donating abilities of 1/15.8 ( = 0.06329) for TMU, 1/16.4 ( = 0.06098) for 1,3DMU and 1/16.9 ( = 0.05917) for 1,1-DMU. It can be seen by comparing these values to v(C=O) in DMSO for each of the three ureas that there is no correlation. Therefore, simple electron donation cannot explain the observed shift in frequency and thus other factors should be taken into account. As can be seen in Fig. 2, 1,1-DMU has lower v(C=O) values than 1,3-DMU. It appears that the amalgamation of one (CH3)2N group and one H2N group is more effective at donating electrons than the combination of two CH3NH groups. In order to explain the difference in v(C=O) between these two molecules, the number of methyl groups on the carbamide fragment as well as their positions and steric interactions must be considered. TMU (with a total of four methyl groups) is able
a~- / t
Fig. 3. Representative resonance structures for alkyl urea derivatives.
to transfer electrons to the C = O group more effectively than 1,1-DMU and 1,3-DMU (with only two methyl groups). TMU has the lowest value of v(C=O) in each of the solvents. This is consistent with the largest degree of delocalization induced by the presence of the four methyl groups and is also consistent with the C = O and C - N stretching motion contributions to the potential energy distributions of the associated vibrational normal modes [11,12,23, 24]. Delocalization processes that occur in urea derivatives are affected by the degree of methyl substitution on the nitrogen atoms in both the cyclic and non-cyclic compounds and can be used to explain the observed order within these groups, As the degree of methyl substitution increases, the nitrogen atoms assume more sp 2 character leading to more planar geometries. This results in increased contributions from the charge separated resonance forms that include greater single-bond character in the carbonyl moiety. An example of possible resonance structures is shown in Fig. 3. Delocalization processes have been considered in computational studies in an attempt to explain the causes leading to the observed v(C=O) differences between EU and DMEU  and in urea derivative-solvent mixtures . It was shown that an increase in the degree of methyl substitution on the nitrogen atoms in both the cyclic and the non-cyclic ureas can be correlated with a decrease in the total population electron density of the C=O bond, with a decrease in the C - O bond length, and with a decrease in the observed v(C=O). Concomitant with the changes occurring in the C= O bond, the presence of methyl groups on the nitrogen atoms leads to C - N bonds with more double-bond character. This effect appears to be more pronounced in the cyclic derivatives than in the non-cyclic derivatives. In addition, delocalization processes lead to an increase in the partial electron density of the oxygen lone pair orbitals and a decrease in the
E.I. Harnagea, P. W. Jagodzinski / Vibrational Spectroscopy 10 (1996) 169-175
partial electron density of the nitrogen atoms lone pair orbitals. In analyzing the dependence of v ( C = O ) on the solvent in the region of the formal non-hydrogen bonding solvents (AN < 30), we observed that the traces in Fig. 2 show similar slopes for all ureas and do not cross. Throughout the region of the formal hydrogen bonding solvents (AN > 30), the trends are similar except for DMEU whose frequencies are larger than those of EU. In all cases, v ( C = O ) decreases with increasing AN and is found at its lowest value in H20, the strongest hydrogen-bonding solvent used in the present study. We believe that for the substituted ureas in this study, the primary interaction with the solvent occurs through the oxygen atom. If the interaction with the solvent occurred through the nitrogen atom lone pair, then the nitrogen atoms would have greater s p 3 character. This would favor the resonance structure with greater C - N single-bond character and greater C = O double-bond character and would result in larger values of v ( C = O ) as the solvent AN increases. Since this behavior is not observed, we considered interactions through the oxygen atom lone pair electrons. If the solvent accepts electrons from the oxygen, the NR 2 groups would donate electrons to the C--O moiety through the C - N group. Such an interaction would favor a resonance form with lower C = O double-bond character, greater C - N doublebond character and lower values of v ( C = O ) as the solvent AN increases. This is what is observed. The increases in v ( C = O ) for the alcohols probably reflect hydrogen bonding that is occurring at the nitrogen atoms in addition to that at the oxygen atom. This effect reverses the trend to lower frequencies with increasing AN. In aqueous solution the interaction is again primarily through the oxygen lone pair electrons due to the strength of water as a hydrogen bond donor and v ( C = O ) is below the values found for the ureas in alcohol solutions. This is consistent with our findings for the ureas in a common solvent. Within each group of ureas (cyclic and non-cyclic) the molecules with the larger number of methyl groups have smaller values of v(C=O). Thus it would appear that methyl substitution hinders solvent attack at the nitrogen atom and promotes attack at the oxygen atom. The strain and resonance factors mentioned above would then control the or-
der of the v ( C = O ) for the ureas and the substituents would control the exact frequencies that are observed in a particular solvent. When we scrutinize the data for each urea in each particular solvent, we note that a few solvents perturb the generally linear patterns (acetone in the case of DMEU and DMSO in the case of 1,3-DMU). The nature of these solvents facilitate a stronger H-bonding interaction with the solute that is reflected in the observed perturbations in the v ( C = O ) values. In the case of TMU in benzene, the perturbation is within the experimental error and is not considered significant. The v ( C = O ) for EU undergoes solvent perturbation. As seen in Fig. 2 (trace A), this frequency does not follow the nearly linear pattern observed for the other ureas. As seen in Table 1, v ( C = O ) for EU in HMPA and benzonitrile are lower (compared to the neighboring frequencies) than expected and are the same (1712.9 cm -1) in nitrobenzene and acetonitrile. This frequency pattern may be explained by considering that EU (Fig. 1) is capable of interacting with solvent molecules through the two - N - H groups within the ring as well as through the carbonyl oxygen. The ring will allow the - N - H moieties to form H-bonds with the solvent more easily than the - N - H moieties in 1,3-DMU and 1,1-DMU. HMPA interacts with EU more strongly than does acetone, thus, the v ( C = O ) for EU in HMPA is lower than that in acetone. Benzonitrile interacts more strongly with EU than does acetonitrile, thus, v ( C = O ) of EU in benzonitrile is lower than that in acetonitrile. Moreover, another indication that complex solutesolvent interactions occur is the presence of additional bands in the C = O region of the spectra recorded for ureas in alcoholic solvents [5,7]. Consequently, the nature of the solvent is especially important in the case in which the solute contains - N - H moieties. Although the traces in Fig. 2 are not parallel, they do exhibit similar solvent dependence indicating that each of the ureas is responding to the changing solvent environment in a similar manner. This is interesting, since we expected that the different conformations and electronic structures of the ureas would have some impact on the solvent-solute interactions. The nearly parallel traces may indicate that the solvents are controlling the urea-solvent interactions.
E.I. Harnagea, P. W. Jagodzinski / Vibrational Spectroscopy 10 (1996) 169-175
5. Conclusions It was found that the presence of the 2-imidazolidinone ring promotes higher values for u(C=O) as a consequence of ring strain and resonance effects for the cyclic ureas compared to the non-cyclic ureas. It was also found that methyl groups attached to the nitrogen atoms promote lower values of u(C=O) by increasing the contribution of the charge separated resonance form. Interestingly, despite the differences that exist among the ureas in this study, they have a generally similar response to the solvent as shown by the correlation between u(C=O) and solvent AN. This may indicate that the solvents are controlling the solvent-solute interactions. During the solvation process, the ureas primarily respond to the solvent by providing electron density that facilitates urea-solvent interactions through the C = O moiety. These interactions may be accompanied by conformational changes due to changes in the sp 3 character about the nitrogen atoms. While molecular orbital methods could provide a better basis for analysis of the electronic structure of the ureas, qualitative resonance correlations also provide considerable insight. The response of ureas to the environment is still not clearly understood. At this time there is sufficient spectroscopic data available to support a more complete molecular orbital study involving the complex dynamics of selected methyl urea derivatives. Such work is important because of the uses of the parent urea and the properties of TMU and DMEU as solvents.
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