摘要:

An experimental and computational investigation of the structure and vibrations of dimethylethylenediamine, a model for poly(ethylenimine) Scott E. Boesch, Shawna S. York, Roger Frech* and Ralph A. Wheeler* Department of Chemistry and Biochemistry, University of Oklahoma, Norman, OK 73019, USA. E-mail: rawheeler@chemdept.chem.ou.edu Received 17th November 2000, Accepted 8th December 2000 Published on the Web 5th January 2001 A combination of hybrid Hartree–Fock/density functional calculations and Raman and IR spectroscopy has been used to perform a vibrational analysis of N,N'-dimethylethylenediamine (DMEDA) and provide band assignments for the experimental spectroscopic data. The structures and vibrational frequencies of several low energy conformations of DMEDA were calculated.The lowest energy structure was found to be TGT with a single intramolecular hydrogen bond. The effect of intramolecular hydrogen bonding on the structures and vibrational frequencies was investigated. Liquid DMEDA is found to be predominantly a mixture of two conformations: TGT with one intramolecular hydrogen bond and TGT with no intramolecular hydrogen bonds. Introduction The potential importance of ionically conducting polymers as electrolytes in a variety of applications, including rechargeable batteries, has stimulated intense interest in these systems.1,2 A wide variety of studies have focused on developing a molecular level understanding of the factors controlling ionic conductivity in poly(ethylene oxide)–salt systems.3–7 Several theoretical and experimental studies have been done on small molecule oligomers of poly(ethylene oxide) (PEO) (glymes) in order to better understand the interactions within these systems.8–13 Fundamental investigations into the nature of interactions in polymer electrolyte systems can be complemented by considering a polymer having a backbone structure similar to that of PEO, but with a non-oxygen hetero-atom.Linear poly(ethylenimine), or PEI, [CH2CH2NH]n, is structurally analogous to PEO, with an N–H group in place of the oxygen. PEI-based electrolytes have been studied by several groups.14–21 In order to understand the polymer–salt interactions in PEI systems, it is first necessary to have knowledge of the vibrational modes of PEI.The simplest model compound for PEI is N,N'-dimethylethylenediamine (DMEDA, Fig. 1). By conducting a complete vibrational analysis of DMEDA, one should be able to better understand the vibrations of poly(ethylenimine) and polymer electrolytes based on a poly(ethylenimine) host. In addition to obtaining the experimental Raman and IR spectra of DMEDA, it is useful to employ computational methods in order to calculate the structures and vibrational frequencies of DMEDA. With the use of hybrid Hartree–Fock/density functional methods, we have studied the vibrations of several different conformations of DMEDA and assigned frequencies in the experimental vibrational spectrum. Fig. 1 DMEDA. DOI: 10.1039/b009250I PhysChemComm, 2001, 1.This journal is © The Royal Society of Chemistry E a a E bE cE ( = + E ) (1- + + +1-c)EVWN X X where E Slater is Slater's local spin density functional for exchange,24 EX Becke is Becke's gradient corrected exchange functional,25 E VWN is the local density correlation functional of Vosko, Wilk and Nusair,26 and EX LYP is the gradient corrected correlation functional of Lee, Yang, and Parr.27 Coefficients giving the relative weights of various approximations for the exchange and correlation energies in this method were optimized to reproduce thermochemical data for a variety of small molecules using a slightly different correlation functional..28 Unless Paper Methodology Experimental N,N'-dimethylethylenediamine (DMEDA) 99% was obtained from Aldrich and stored in a dry nitrogenatmosphere glovebox.Infrared spectra were recorded with a Bruker IFS66V FT-IR over a range of 4000 to 500 cm –1 at a resolution of 1 cm –1. The DMEDA liquid was put between ZnSe plates in a sealed sample holder in the glovebox. The IR spectra were taken under dry air purge. Raman spectra were collected using a Jobin-Yvon T64000 Raman spectrometer with a CCD detector. The 514.5 nm line of an argon ion laser at a power of 300 mW at the laser head was used for excitation. Spectra were collected in a 90o scattering geometry. Raman samples were sealed in cuvettes inside the glovebox. Curve fitting analysis was accomplished using a commercial program (Galactic Grams version 5.05).Spectra were curve-fitted to a straight base line and one Gaussian–Lorenzian product function for each band using a non-linear least squares method. Computational The B3LYP hybrid Hartree–Fock/density functional method22,23 was used to perform complete geometry optimizations and vibrational frequency calculations on dimethylethylenediamine. The three-parameter HF/DF method employs a weighted sum of Hartree–Fock (EX HF), local DF, and gradient corrected DF expressions for the exchange and correlation energies as in the following equation: Slater HF Becke LYP X X X C Cotherwise noted, all calculations reported here were performed using the 6-31G(d) split-valence plus polarization basis set.29 This basis set was chosen because it accurately reproduces the structures and vibrational spectra of medium-sized organic molecules and is small enough for rapid calculations.1 The quantum chemistry program GAUSSIAN9423 was used for all calculations. Berny’s optimization algorithm30 was used to perform full geometry optimizations in C symmetry. It was discovered that DMEDA(0) and DMEDA(2) have a 2-fold rotation axis and they were therefore reoptimized in C2 symmetry. Harmonic frequency calculations were performed at the optimized geometries without correcting for anharmonicity. It has become customary to scale calculated frequencies to facilitate comparisons with experiment and we chose to use the multiplicative scaling factors of Scott and Radom.31 All frequencies less than 1000 cm–1 are multiplied by 1.0013 and all frequencies greater than 1000 cm–1 are multiplied by 0.9614.Vibrational mode assignments were performed by animating each mode using the program XMOL32 and comparing the modes of one geometry to another using the program ViPA, an acronym for Vibrational Projection Analysis.33–35 The ViPA program exploits the vector properties of vibrational normal modes to assess the similarity between modes of an object molecule and a structurally similar basis molecule. The program first aligns the two molecules and calculates each molecule’s normal modes and vibrational frequencies. For each molecule, each of the normal vibrational modes is a column vector, which is orthonormal to all other normal modes of the same molecule.The vector projection operation is done by Fig. 2 The TGT conformations of DMEDA. (a) DMEDA(0), (b) DMEDA(1), (c) DMEDA(2). Click on the images or here to view 3D structures. Fig. 3 Newman projections of DMEDA with methyl groups deleted. (a) DMEDA(0), (b) DMEDA(1), (c) DMEDA(2). Click on the images or here to view 3D structures. sequentially projecting each normal mode of the object molecule on the modes of the basis molecule. The similarity of any mode of the object molecule to any mode of the basis molecule can then be expressed as a percentage by calculating the sum of the squares of the matrix elements and multiplying by 100. Vibrational projection analysis has been used to compare normal modes modified by isotopic or chemical substitution, oxidation–reduction and non-covalent contacts.33–36 Computational results Geometry optimizations were performed on DMEDA and the lowest energy structure was determined to have a TGT conformation, signifying that the first torsional angle (C–N–C–C) has a trans conformation corresponding to an angle of 180 ± 60º, T, the second torsional angle (N–C–C–N) has a gauche conformation corresponding to a torsional angle 60 ± 60º, G, and the third torsional angle (C–C–N–C) also adopts a trans conformation.There are three possible structures of DMEDA having TGT conformations. These will be henceforth referred to as DMEDA(0), DMEDA(1), and DMEDA(2), (see Fig. 2a–c) with their main differences being in the central N–C–C–N torsional angle and the presence of intramolecular hydrogen bonding.DMEDA(0) has no hydrogen bonding, DMEDA(1) has one hydrogen bonding interaction and DMEDA(2) has two hydrogen bonding interactions. Fig. 3a–c show Newman projections along the central C–C bond with the methyl groups deleted. DMEDA(1) and DMEDA(2) are the lowest energy structures of eight possible conformations of DMEDA,while the next highest in energy are TTT, GGG, TTG, DMEDA(0), TTG and GTG. The energy difference between the lowest energy, DMEDA(1) structure and the highest energy DMEDA(0) structure is 3.13 kcal mol –1. DMEDA(2) is only 1.53 kcal mol –1 higher in energy than DMEDA(1). DMEDA(0) and DMEDA(2) have C symmetry and are more symmetrical than DMEDA(1).Table 1 shows the relative energies and dipole moments for low energy conformations of DMEDA. Calculated bond distances, bond angles, and torsional angles for the three TGT conformations of DMEDA are presented in Table 2. The C–N bond distances in these three conformations range from 1.455–1.464 Å, slightly smaller than the experimentally determined C–N bond distance for methylamine, 1.47 Å.37 The C–C Table 1 Comparison of relative energies and dipole moments for eight conformations of DMEDA Energy/kcal mol–1 Dipole moment/D DMEDA(1) DMEDA(2) TTT GGG TTG DMEDA(0) TGG GTG aThe actual total energy for DMEDA(1) is calculated to be –269.13073 Eh. Table 2 Comparison of the bond distances (Å), selected bond angles (°), and torsional angles (°) for the three TGT conformations of DMEDA DMEDA(0) C2–N5 C2–H1 C2–H3 C2–H4 N5–C6 C6–H7 C6–H8 C6–C9 C9–H10 C9–H11 C9–N12 N12–C13 C13–H14 C13–H15 C13–H16 N5–H17 N12–H18 C2–N5–C6 N5–C6–C9 C6–C9–N12 C9–N12–C13 C9–N12–H17 C6–N5–H18 C2–N5–C6–C9 N5–C6–C9–N12 C6–C9–N12–C13 2 0 a 1.54 2.06 2.10 2.93 3.13 4.00 4.27 1.455 1.095 1.097 1.108 1.456 1.102 1.109 1.530 1.109 1.102 1.456 1.455 1.095 1.097 1.108 1.017 1.017 113.3 112.6 112.6 113.3 109.7 109.7 166.0 46.7 166.0 bond distance for DMEDA(0) and DMEDA(2) is calculated to be 1.530 Å, while the C–C bond distance for DMEDA(1) is 1.527 Å.These are very comparable to the C–C single bond distance of 1.54 Å in ethane.All calculated C–H distances in the three conformations are quite similar and they range from 1.095 to 1.112 Å, close to the C–H distance in methane, 1.11 Å. The calculated N–H distances for these conformations range from 1.017 to 1.020 Å. The calculated bond angles of the two conformations are unremarkable. The calculated �C–N–C range from 113.4º to 113.1º, greater than the �C–N–C value in trimethylamine of 108.7º. DMEDA(1) 1.459 1.095 1.096 1.106 1.464 1.097 1.107 1.527 1.099 1.112 1.455 1.454 1.095 1.108 1.097 1.020 1.019 113.4 110.4 110.4 113.4 108.5 107.3 185.8 62.9 189.3 0.941 0.043 0.000 2.042 1.379 0.941 1.256 1.910 DMEDA(2) 1.460 1.095 1.096 1.106 1.463 1.106 1.098 1.530 1.098 1.106 1.463 1.460 1.095 1.096 1.106 1.019 1.019 113.1 111.1 111.1 113.1 107.4 107.4 169.9 53.2 169.9For DMEDA(0), the C–C–N–C angles have a value of 166.0º and the N–C–C–N angle is 46.7º.For DMEDA(2) the C–C–N–C angles are 169.9º and the N–C–C–N angle is 52.3º. The H–N–C–C angles both have values of 42.0º for DMEDA(0) and –70.9º for DMEDA(2). For DMEDA(1), one side of the molecule has a C–N–C–C angle of 185.8º and the other side of the molecule has a C–N–C–C angle of 189.3º. The central N–C–C–N angle is 62.9º. The H–N–C– C angles are 64.5 and –48.2º. A possible reason for the structural differences between DMEDA(0), DMEDA(1) and DMEDA(2) is intramolecular hydrogen bonding (N–H···N).In the structure of DMEDA(0), the distance from each nitrogen to the Table 3 Scaled31 calculated vibrational frequencies (cm–1), mode assignments, and IR intensities for DMEDA(0) Mode assignment Methyl wag; N–H bend NCCN torsion; methyl wag Methyl wag Methyl twist Methyl twist Methyl twist; CH2 rock C–N–C bend; (out-of-phase) C–N–C bend (in-phase) C–N–C bend (out-of-phase); CH2 twist N–H par bend (out-of-phase) N–H par bend (in-phase) CH2 rock; CH2 twist N–H par bend (in-phase); CH2 twist; methyl wag Methyl wag; CH2 wag C–C stretch CH2 rock; N–H parallel bend Methyl wag; N–H parallel bend Methyl wag; CH2 twist CN stretch (out-of-phase) CN stretch (in-phase) C–C stretch Methyl wag; CH2 rock CH2 rock; methyl symmetric deformation CH2 twist CH2 twist; methyl symmetric deformation CH2 wag CH2 wag Methyl symmetric deformation (in-phase) Methyl symmetric deformation (out-of-phase) N–H parallel bend; methyl antisymmetric deformation N–H parallel bend; methyl antisymmetric deformation Methyl antisymmetric deformation N–H parallel; methyl antisymmetric deformation CH2 scissors; methyl antisymmetric deformation Methyl antisymmetric deformation CH2 scissors; methyl antisymmetric deformation CH2 scissors CH2 stretch CH2 stretch; CH3 stretch CH3 stretch CH3 stretch; CH2 stretch CH2 stretch CH2 stretch CH3 stretch; CH3 stretch CH3 stretch CH3 stretch; CH3 stretch CH3 stretch; CH3 stretch N–H stretch (out-of-phase) N–H stretch (in-phase) opposite amine hydrogen is 2.854 Å.In DMEDA(1), the distances from a nitrogen to the opposite amine hydrogen are 2.444 and 3.306 Å. In DMEDA(2), the distance from each nitrogen to the opposite amine hydrogen is 2.583 Å. So DMEDA(0) has no intramolecular hydrogen bonds, DMEDA(1) has one intramolecular hydrogen bond and DMEDA(2) has two intramolecular hydrogen bonds. Typically hydrogen bonds are linear and display shorter N– H···N distances; however, this is not always the case, especially in cases of intramolecular hydrogen bonding where the geometry of the molecule forbids it.38 In the case of DMEDA(1) the N–H···N angle is 107.5º, while for DMEDA(2) the angle is 95.0º. Although the hydrogen Mode Sym.Frequency 59 86 12 BAA 125 221 34 AB 257 284 56 AB 313 340 78 AB 564 727 910 BA 752 892 11 12 902 990 13 14 BAB 1010 1022 15 16 1092 1112 17 18 AABA 1115 1144 19 20 BA 1153 1195 21 22 1251 1255 23 24 BAABB 1353 1359 25 26 A 1428 1429 27 28 ABAB 1448 1450 29 30 1459 1462 31 32 1482 1485 33 34 1493 1497 35 36 2799 2809 37 38 2824 2826 39 40 2887 2905 41 42 2945 2946 43 44 ABBABABABAABBABA 2995 2996 45 46 3373 3373 47 48 BA IR intensity 63002113357 153 60012 00276 23 200733 10510 38 3722 8117116 193 96 794 80 15 30 41 00 bonds in DMEDA are probably weaker than conventional hydrogen bonds, it is not unreasonable to conclude that the hydrogen bond in DMEDA(1) is stronger than the hydrogen bonds in DMEDA(2), since the total energy for DMEDA(1) is lower and the hydrogen bonding distance is shorter than DMEDA(2). The energy difference between the non-hydrogen-bonded structure, DMEDA(0) and the singly hydrogen bonded structure, DMEDA(1), is 3.13 kcal mol–1.N–H···N bonds typically have energies of 2–6 kcal mol–1.38,39 This also implies that the energy difference between DMEDA(0) and DMEDA(1) may be due to the presence of hydrogen bonding.Calculated vibrational modes Each of the conformations of DMEDA has 48 vibrational modes. We will examine in detail the vibrations of the three TGT conformations. Emph will be placed on the modes that have the highest infrared intensities, especially those that differ significantly for different conformations, since we can qualitatively compare these with experiment. Table 3 presents the scaled calculated vibrational frequencies, IR intensities, and mode assignments for DMEDA(0). DMEDA(0), possessing no hydrogen bonding, has a 2-fold rotation axis through the central C–C bond. The lowest vibrational frequency of DMEDA(0) is at 59 cm–1 and is assigned to a mixture of methyl wagging and N–H bending. The next highest frequency at 86 cm–1 also has some methyl wagging as well as torsional motion about the N–C–C–N angle.The next four modes, calculated at 125, 221, 257 and 284 cm–1, have the preponderance of the motion concentrated on of the methyl groups. We then have calculated frequencies involve bending of the C–N–C angles. These modes can be divided into in-phase (340 cm–1) and out-of-phase (313 and 564 cm–1) groups. Calculations show that there are three modes with significant contributions from N–H bending perpendicular to the long axis of the molecule occurring at 727, 752 and 902 cm–1. The modes involving CH2 rocking are calculated to occur at 892, 1022 and 1195 cm–1. The C–C stretch is calculated at 1009 cm–1 and the calculated C–N stretches are at 1115 cm–1 (out-ofphase) and 1144 cm–1 (in-phase).In the region from 1150– 1500 cm–1, calculations show a plethora of different types of modes involving the hydrogens. We calculate modes that have primarily methyl wagging at 1153 cm–1, CH2 twisting at 1251 and 1255 cm–1, and CH cm–1. The symmetric methyl deformations are at 1428 and 1429 cm–1 and there are two modes assigned to methyl antisymmetric deformations at 1459 and 1485 cm–1. Three modes involving N–H bending parallel to the axis of the molecule are at 1448, 1450 and 1462 cm–1. The modes primarily assigned to scissoring of the CH 1482, 1493 and 1497 cm–1. There are ten calculated frequencies that are C–H stretches and the two highest vibrational frequencies are, of course, N–H stretches.The modes for the N–H stretches are also separated into inphase (3373 cm–1) and out-of-phase (3373 cm–1) motions. 2 wagging at 1353 and 1359 2 groups are at Table 4 shows the scaled, calculated vibrational frequencies, mode assignments, and infrared intensities of DMEDA(1). The mode assignments for the vibrational frequencies of DMEDA(1) are, in general, quite similar to the assignments of DMEDA(0). However, due to the lowering of the symmetry, we see instances in which a particular motion that was previously associated with both ends of the molecules in DMEDA(0) is now concentrated only on one particular end of the molecule. Instances of this will be pointed out in the following discussion of the vibrational modes of DMEDA(1).The two lowest calculated vibrational frequencies of DMEDA(1) are at 91 and 101 cm–1 and have been assigned to methyl wagging. The next four modes, 165, 244, 251 and 277 cm–1 involve mostly methyl twisting. We have one mode (359 cm–1) containing large contributions from inphase C–N–C bending and two modes (316 and 564 cm–1 ) containing significant amounts of out-of-phase C–N–C bending. The major contributions for the next five modes, occurring at 788, 825, 904, 912 and 984 cm–1 are N–H bending perpendicular to the axis of the molecule. The calculated C–C stretch for this conformation is at 999 cm–1, lower by 10 cm–1 than the C–C stretch for DMEDA(0). There are three calculated modes in which the C–N stretching motion has the greatest contribution. At 1098 cm–1 the C–N stretching is concentrated on one end of the molecule, at 1115 cm–1 the C–N stretching is concentrated on the opposite end of the molecule, and at 1134 cm–1 the C–N stretching motion is approximately equally distributed on each end of the molecule.The calculated frequencies between 1150–1500 cm–1 are again a mixture of many different types of motions involving the hydrogens. The modes at 1342 and 1362 cm–1 are mostly methyl wagging. The symmetric methyl deformations are calculated at 1425 and 1428 cm–1 . This is noteworthy because each of these modes is mostly concentrated on one particular methyl group. There is primarily N–H bending parallel to the axis of the molecule in the modes at 1444 and 1451 cm–1 and CH2 scissoring very definitely predominates in the modes at 1476 and 1491 cm–1.The antisymmetric methyl deformations are at 1459, 1461, 1482 and 1486 cm–1. The ten calculated C–H stretches are similar to those in DMEDA(0) and they occur between 2771 and 3000 cm–1. Unlike in DMEDA(0), the N–H stretches are each concentrated on one end of the molecule and occur at different frequencies (3338 and 3363 cm–1 ). Table 5 shows the scaled harmonic vibrational 2 twisting. The mode at 1111 cm–1 has CH2 rocking, frequencies, mode assignments, and infrared intensities calculated for the TGT conformation of DMEDA that has two hydrogen bonds, DMEDA(2). The vibrational modes are most similar to those of DMEDA(0), since DMEDA(2) also has C2 symmetry.The lowest calculated vibrational frequency for DMEDA(2) is assigned to methyl wagging and is calculated at 66 cm–1. The next highest frequency, at 101 cm–1, has been assigned to an N–C–C–N torsion. There is methyl wagging at 151 cm–1 and then methyl twisting at 214, 250, and 299 cm–1. The out-of-phase C–N–C bend is at 320 cm–1 and the in-phase C–N–C bending is at 342 cm–1. The mode at 559 cm–1 is comprised of CH2 twisting and C–N–C bending. There are out-of-phase and in-phase perpendicular bending modes of the N–H groups at 818 and 835 cm–1, respectively. A mode at 853 cm–1 is a combination of CH2 rocking and N– H perpendicular bending and modes at 945 and 976 cm–1 are a mixture of N–H perpendicular bending and methyl wagging.The C–C stretch is calculated at 1006 cm–1. The mode calculated at 1036 cm–1 is assigned to CH2 rocking and N–H parallel bending. The out-of-phase and in-phase C–N stretches are calculated at 1089 and 1138 cm–1, respectively. There is a calculated mode at 1104 cm–1 that is assigned to a combination of methyl wagging and CH methyl wagging, and CH2 twisting character. The modes calculated at 1162, 1183 and 1256 cm–1 are assigned to methyl wagging and CH2 twisting. A mode at 1287 cm–1 is primarily CH2 twisting, while the modes at 1347 and 1370 cm–1 are mostly CH2 wagging. The two modes calculated at 1425 and 1426 cm–1 are both assigned to symmetric methyl deformations. Modes assigned to N– H parallel bending mixed with methylTable 4 Scaled31 calculated vibrational frequencies (cm–1), mode assignments, and IR intensities for DMEDA(1) Mode Frequency IR intensity Mode assignment Methyl wag Methyl wag Methyl twist; CH2 rock Methyl twist Methyl twist Methyl twist; CH2 rock C–N–C bend (out-of-phase) C–N–C bend (in-phase) C–N–C bend (out-of-phase) N–H par bend; CH2 rock; CH2 twist N–H par bend; CH2 rock N–H par bend; CH2 rock; CH2 twist 2 wag; methyl wag N–H par bend (12); CH N–H parallel bend (12); methyl deformation (antisymmetric) C–C stretch N–H parallel bd (5); CH2 rock: methyl deformation (antisymmetric) C–N stretch (2,5) Methyl deformation (antisymmetric) (13); CH2 twist C–N stretch; methyl deformation (antisymmetric); CH2 twist C–N stretch CH2 rock; methyl deformation (antisymmetric) CH2 rock; methyl wag CH2 twist; CH2 rock CH2 rock; CH2 twist CH2 wag CH2 wag Methyl deformation (symmetric) (2) Methyl deformation (symmetric) (13) N–H par bend (5); CH2 scissors N–H par bend; methyl def (a); CH2 wag Methyl deformation (antisymmetric) Methyl deformation (antisymmetric); N–H parallel bend CH2 scissors Methyl deformation (antisymmetric); CH2 scissors Methyl deformation (antisymmetric) CH2 scissors CH2 stretch CH3 stretch CH2 stretch; CH3 stretch CH2 stretch; CH3 stretch CH2 stretch CH3 stretch CH2 stretch; CH3 stretch CH2 stretch; CH3 stretch CH3 stretch CH3 stretch N–H stretch 3363 N–H stretch 23020331084 77 15 23 1611 30 441 23 843920 37415 34 9517 14 8486 129 61 138 44 57 19 48 37 33 17 1 91 2 101 3 165 4 244 5 251 6 277 7 316 8 359 9 564 10 788 11 825 12 904 13 912 14 984 15 999 16 1022 17 1098 18 1102 19 1115 20 1134 21 1163 22 1185 23 1248 24 1268 25 1342 26 1362 27 1425 28 1428 29 1444 30 1451 31 1459 32 1461 33 1476 34 1482 35 1486 36 1491 37 2771 38 2822 39 2831 40 2842 41 2930 42 2944 43 2957 44 2958 45 2993 46 3000 47 3338 48 antisymmetric deformation appear at 1437, 1454 and 1488 cm–1 and there are modes assigned exclusively to methyl antisymmetric deformation at 1460, 1461 and 1484 cm–1. The CH2 scissoring modes occur at 1471 and 1475 cm–1.The next ten highest modes are C–H stretches and the two highest modes are N–H stretches, occurring at 3347 (in-phase) and 3353 cm–1 (out-ofphase). A comparison of Tables 3, 4 and 5 shows several C–N stretching or C–N–C bending change significantly in frequency between the three conformations. In contrast, modes involving the N–H bend parallel to the axis of the molecule, methyl deformations, C–C stretch or CH2 scissors show little or no frequency change between the different conformations. The N–H stretches are lower in frequency in the hydrogen-bonded conformations than in DMEDA(0), as is expected; however, the frequency of the hydrogen-bonded N–H stretch of DMEDA(1) is higher (3363 cm–1) than one would expect from comparison with DMEDA(2).interesting trends. Modes involving N–H bending perpendicular to the axis of the molecule, CH2 rocking,Table 5 Scaled31 calculated vibrational frequencies (cm–1 ), mode assignments, and IR intensities for DMEDA(2) Mode assignment Methyl wag NCCN torsion Methyl wag Methyl twist Methyl twist Methyl twist; CH2 rock CNC bend (out-of-phase) CNC bend (in-phase) CH2 twist; CNC bend N–H par bend (out-of-phase) N–H par bend (in-phase) CH2 rock; N–H par bend N–H par bend; methyl wag N–H par bend; methyl wag C–C stretch CH2 rock; N–H par bend C–N stretch (out-of-phase) Methyl wag; CH2 twist CH2 rock; methyl wag; CH2 twist C–N stretch (in-phase) Methyl wag; CH2 twist Methyl wag; CH2 twist CH2 twist; methyl wag CH2 twist CH2 wag CH2 wag Methyl symmetric deformation (out-of-phase) Methyl symmetric deformation (in-phase) N–H par bend; methyl antisymmetric deformation N–H par bend; methyl antisymmetric deformation Methyl antisymmetric deformation Methyl antisymmetric deformation CH2 scissors; methyl antisymmetric deformation CH2 scissors Methyl antisymmetric deformation N–H par bend; methyl antisymmetric deformation CH2 stretch CH3 and CH2 stretch CH3 and CH2 stretch CH3 and CH2 stretch CH2 stretch CH2 stretch CH3 stretch CH3 stretch CH3 stretch CH3 stretch N–H stretch N–H stretch Comparison to experimental bands The assignment of the calculated modes to the experimental bands was done by comparing calculated frequencies and intensities to experimental IR and Raman frequencies and IR intensities.The IR and Raman spectra of DMEDA are shown in Fig. 4. Since the experimental frequencies obtained are in the liquid state, it is highly probable that we are not simply dealing with a single conformation, but several different conformations. Table 6 shows the proposed mode assignments, calculated frequencies of all three TGT conformations, and experimental Raman and infrared frequencies for DMEDA. The lowest frequencies present in the Raman spectrum Mode Sym. Frequency 66 101 12 151 214 34 250 299 56 320 342 78 559 818 910 835 853 11 12 945 976 13 14 1006 1036 15 16 1089 1104 17 18 1111 1138 19 20 1162 1183 21 22 1256 1287 23 24 1347 1370 25 26 1425 1426 27 28 1437 1454 29 30 1460 1461 31 32 1471 1475 33 34 1484 1488 35 36 2834 2838 37 38 2841 2848 39 40 2946 2947 41 42 2953 2954 43 44 2997 2997 45 46 3347 3353 47 48 BAAABABABABBABAABABAABBABABABABABABAABABBABABABA occur at 243 and 268 cm–1.Calculations give us frequencies of 221 and 257 cm–1 for DMEDA(0), 244 and 251 cm–1 for DMEDA(1), and 214 and 250 cm–1 for DMEDA(2). These calculated frequencies have all been assigned to a methyl twist.There are bands measured at 345, 360, 395, 425, 542 and 559 cm–1 in the Raman spectrum. The calculations for the three conformations of DMEDA give frequencies of 313, 340 and 564 cm–1 for DMEDA(0), 316, 359 and 564 cm–1 for DMEDA(1), and 320, 342 and 559 cm–1 for DMEDA(2). They are mostly C–N–C bending and correspond most closely to the Raman frequencies at 345, 360, and 559 cm–1. IR intensity 71101541058 119 510 92144 258 412 517 223 512 028 322 017 46717 131 12 198 36 41 95 1367 00Table 6 Comparison of the scaled31 calculated and experimental frequencies (cm–1) of DMEDA Mode assignments Methyl twist C–N–C bend N–H ^ bend CH2 rock Methyl wag C–C stretch CH2 rock C–N stretch CH2 twist CH2 wag Methyl sym.def. N–H bend, methyl as sym. def. CH2 scissors, methyl as sym. def. C–H stretch N–H stretch Fig. 4 Infrared and Raman spectra of DMEDA. Click on particular peaks of the spectra to view either a blowup of that region or an animated vibrational mode. Fig. 5a shows the IR and Raman spectra from 950 to 650 cm–1. There is a broad band present at 805 cm–1 in the Raman spectrum and bands at 789 and 736 cm–1 in the infrared spectrum. These are analogous to the calculated frequencies for DMEDA(0) at 727 and 752 cm–1, DMEDA(1) at 788 cm–1 , and DMEDA(2) at 818 cm–1 which have been assigned to N–H bending. The Raman DMEDA(0) DMEDA(2) DMEDA(1) IR 214 250 221 257 320 342 313 340 559 564 727 752 818 835 853 976 892 990 1006 1036 1010 1022 1089 1138 1115 1144 1256 1347 1255 1353 1370 1425 1359 1428 1426 1437 1429 1448 1450 1462 1454 1460 1482 1485 1471 1488 2809 2824 2838 2848 2826 2905 2946 2947 2946 2996 2953 2997 3347 3353 3373 3373 frequency at 881 cm–1 and the infrared frequency at 879 cm–1 have been assigned to a CH2 rock since the calculated frequency for DMEDA(0) is at 892 cm–1, for DMEDA(1) it appears at 904 cm–1, and for DMEDA(2) it appears at 853 cm–1.The C–C stretch, which calculations show at 1010 cm–1 for DMEDA(0), 999 cm–1 for DMEDA(1), and 1006 cm–1 for DMEDA(2) occurs in the infrared spectrum at 1022 cm– 1 and in the Raman spectrum at 1021 cm–1.A mode calculated to occur at 1022 cm–1 for both DMEDA(0) and DMEDA(1) and at 1036 cm–1 for DMEDA(2) has been assigned to a mixture of CH2 rock and N–H bend; this mode appears to correspond to the band measured in the Raman spectrum at 1039 cm–1 and in the infrared spectrum at 1042 cm–1. The calculated and experimental infrared intensities of this mode are both quite low. The C–N stretch calculated 1115 and 1144 cm–1 for DMEDA(0), 1098, 1115 and 1134 cm–1 for DMEDA(1), and 1089 and 1138 cm–1 for DMEDA(2) all have moderately high infrared intensity. These correspond to the infrared frequencies observed at 1106, 1122 and 1151 cm–1 and the Raman frequencies observed at 1113, 1124 and 1152 cm–1.The infrared frequency at 1251 cm–1 and the Raman frequency at 1255 cm–1 have been assigned to mostly CH Raman 243 268 244 251 345 360 316 359 395 425 564 542 559 736 789 805(b) 788 825 881 990 879 985 904 984 1021 1039 1022 1042 999 1022 1106 1122 1115 1134 1113 1124 1152 1255 1151 1251 1248 1342 1347 1363 1346 1361 1362 1425 1420 1418 1428 1444 1450 1444 1451 1459 1473 1473 1476 1482 2771 2822 2681 2788 2679 2790 2831 2842 2840 2887 2843 2891 2944 2958 2934 2967 2939 2970 3290 3327 3280 3323 3338 3363 2Fig. 5 (a) Infrared and Raman spectra of DMEDA in the CH2 rocking and NH bending region.(b) Infrared and Raman spectra of DMEDA in the N–H stretching region. Click on particular peaks of the spectra to view either a blowup of that region or an animated vibrational mode. twisting. The calculated frequencies occur at 1255 cm–1 for DMEDA(0), 1248 cm–1 for DMEDA(1), and 1256 cm–1 for DMEDA(2). The IR frequencies at 1346 and 1361 cm–1 and the Raman frequencies at 1347 and 1363 cm–1 have all been assigned to CH2 wagging. These are most similar to the calculated modes at 1353 and 1359 cm–1 for DMEDA(0), 1342 and 1362 cm–1 for DMEDA(1), and 1347 and 1370 cm–1 for DMEDA(2). We assign the IR frequency at 1418 cm–1 and the Raman frequency at 1420 cm–1 to a symmetric deformation of the methyl group. The calculated frequency is at 1428 cm–1 for both DMEDA(0) and DMEDA(1) and 1426 cm–1 for DMEDA(2).The IR frequency at 1444 cm–1, which is comparable to the Raman frequency 1450 cm–1, is a mixture of N–H bending and an antisymmetric deformation of the methyl group, with a calculated frequency of 1450 cm–1 for DMEDA(0). The bands present at 1473 cm–1 in both the Raman and the infrared are a mixture of CH2 scissoring and antisymmetric deformation of the methyl group and correspond most closely to a calculated frequency of 1482 cm–1 for DMEDA(0). The bands present in the 2600–3000 cm–1 range are C–H stretches. The highest frequency modes are N–H stretches. The IR and Raman spectra for the N–H stretching region are shown in Fig. 5b. The calculated frequencies for the N–H stretches are important because they support our statement that DMEDA(1) possesses some intramolecular hydrogen bonding, as hydrogen bonding causes the frequencies to be lowered.38 It has been shown in the literature that DMEDA without hydrogen bonding has an N–H stretch of 3371 cm–1 and DMEDA with hydrogen bonding has an N–H stretch of 3341 cm–1.40 In DMEDA(0) the N–H stretch has a calculated frequency of 3374 cm–1, while in DMEDA(2) the N–H stretches lower to 3347 and 3353 cm –1 and for DMEDA(1) the N–H stretches are 3338 and 3363 cm–1.Preliminary calculations with two DMEDA molecules having an intermolecular hydrogen bond predict the intermolecular hydrogen bonded N–H stretching frequencies to be 3251 cm–1. The N–H stretching frequencies in the IR spectrum are 3323 and 3280 cm–1, and in the Raman spectrum are 3327 and 3290 cm–1, which indicates hydrogen bonding is present in liquid DMEDA.Since DMEDA is a liquid, it is highly probable that several different conformations contribute to the experimental spectra. The modes most sensitive to conformational change are the N–H perpendicular bend and CH2 rock (shown in Fig. 4), and the C–N stretch. As seen in Table 5, the IR and Raman frequencies in these regions are closest to those calculated for DMEDA(0) and DMEDA(1). In the N–H bending region, the IR spectrum has frequencies of 736 and 789 cm–1, in good agreement with the 727 cm–1 of DMEDA(0) and the 788 cm–1 of DMEDA(1). The Raman spectrum in this region has a very broad band around 805 cm–1, which may contain underlying bands at frequencies ranging from ~760 to 805 cm–1.In the CH2 rocking region DMEDA(0) has a calculated frequency of 892 cm–1 and DMEDA(1) has a frequency of 904 cm–1 , both of which are closer to the experimental IR and Raman frequencies of 879 and 881 cm–1 than are those calculated for other conformations. The frequencies in DMEDA(2) of 818 and 835 cm–1 for the N– H bend are too high and the frequency of 853 cm–1 for the CH2 rock is too low compared to the IR and Raman spectra. Also notable are the C–N stretching frequencies, for which again the DMEDA(0) and DMEDA(1) frequencies are quite close to the experimental frequencies but the DMEDA(2) frequency of 1089 cm–1 is not comparable with any experimental band.Comparison was also made to the five other conformations of DMEDA (not discussed in detail in this paper), none of which showed as good an agreement between calculated and experimental frequencies and intensities as DMEDA(1) and DMEDA(0) in these regions. For example, the TTT conformation has frequencies of 837 cm–1 for CH2 rocking and 972 and 975 cm–1 for C–N stretching, none of which are close to any band in the experimental IR or Raman spectra. We therefore conclude that liquid DMEDA appears to consist of primarily a mixture of DMEDA(1) and DMEDA(0), with smaller contributions from other conformations. It is important to note that in the liquid DMEDA, intermolecular as well as intramolecular hydrogen bonding is possible, which may contribute to the stabilization of higher-energy conformations.Indeed our analysis of experimental vibrational mode frequencies strongly argues that such stabilization occurs in this system. Conclusions Hybrid Hartree–Fock density functional calculations were performed on N,N'-dimethylethylenediamine, DMEDA, and it was found that the lowest energy conformation is TGT with one intramolecular hydrogen bond. The major differences between these three conformations are in the N–C–C–N torsional angle and the extent of hydrogen bonding. The conformations DMEDA(0) and DMEDA(2) both have C2 symmetry but the presence of the single hydrogen bond in DMEDA(1) reduces it to C1 symmetry. The non-covalent N–H···N distance in DMEDA(1) is 2.444 Å and the non-covalent N–H···N distances in DMEDA(2) are both 2.853 Å.Notable mode frequency differences between DMEDA(0) and DMEDA(2) occur in those modes whosefrequencies are most affected by hydrogen bonding. The C–N stretches and the N–H stretches are lower in frequency in DMEDA(2) than DMEDA(0), probably due to hydrogen bonding. For similar reasons, the N–H bends are higher in frequency in DMEDA(2) than DMEDA(0). The differences in the mode assignments between DMEDA(0) and DMEDA(1) due primarily to the single intramolecular hydrogen bond in DMEDA(1) causing the loss of symmetry. Compared to DMEDA(0), one of the C–N stretches remains the same and the other decreases in frequency and the N–H bends increase in frequency. The N–H stretches decrease in frequency compared to DMEDA(0), again due to hydrogen bonding.The assignment of the experimental infrared and Raman frequencies of DMEDA to specific vibrational modes was done using the calculated assignments. The calculated and experimental frequencies are generally in quite good agreement and IR intensities are similar. In order to determine the conformations present in liquid DMEDA, we have compared the modes sensitive to conformational change to the experimental spectra. We conclude that DMEDA appears to be primarily a mixture of DMEDA(1) and DMEDA(0). In the N–H stretching region, the presence of two bands with a large frequency separation is also consistent with a mixture of conformations.The unusually low frequencies of one of the N–H stretches in the experimental data (3280 and 3290 cm–1 for the IR and Raman), compared with preliminary calculations on two DMEDA molecules with an intermolecular hydrogen bond, indicate the presence of intermolecular as well as intramolecular hydrogen bonding in DMEDA liquid. Further work is necessary to probe the structures and quantify the extent of intermolecular vs. intramolecular hydrogen bonding in liquid DMEDA. Acknowledgements The research described in this publication was made possible by a grant to R.F. from the National Science Foundation (DMR-0072544), and grants of supercomputer time to R.W. from the NSF/NCSA. 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ISSN:1460-2733    

DOI:10.1039/b009250i

出版商:RSC    

年代:2001

数据来源: RSC