throbber
Conformational Equilibrium and Hydrogen Bonding in Liquid
`2-Phenylethylamine Explored by Raman Spectroscopy and
`Theoretical Calculations
`
`Min Xie, Yajing Qi, and Yongjun Hu*
`
`MOE Key Laboratory of Laser Life Science & Institute of Laser Life Science, College of Biophotonics,
`South China Normal University, Guangzhou 510631, China
`bS Supporting Information
`
`ABSTRACT: 2-Phenylethylamine (PEA) is the simplest aro-
`matic amine neurotransmitter, as well as one of the most
`important. In this work, the conformational equilibrium and
`hydrogen bonding in liquid PEA were studied by means of
`Raman spectroscopy and theoretical calculations (DFT/MP2).
`By changing the orientation of the ethyl and the NH2 group,
`nine possible conformers of PEA were found, including four
`degenerate conformers. Comparison of
`the experimental
`Raman spectra of liquid PEA and the calculated Raman spectra
`of the five typical conformers in selected regions (550800 and 12501500 cm
`1) revealed that the five conformers can coexist in
`1 relative to that of an
`conformational equilibrium in the liquid. The NH2 stretching mode of the liquid is red-shifted by ca. 30 cm
`isolated PEA molecule (measured previously), implying that intermolecular NH 3
`3
`3 N hydrogen bonds play an important role in
`1 was found to increase with increasing temperature, indicating that
`liquid PEA. The relative intensity of the Raman band at 762 cm
`the anti conformer might be favorable in liquid PEA at room temperature. The blue shift of the band for the bonded NH stretch
`with increasing temperature also provides evidence of the existence of intermolecular NH 3
`3
`3 N hydrogen bonds.
`
`1. INTRODUCTION
`
`Raman scattering spectroscopy, which identifies substances
`from their characteristic spectral patterns (fingerprinting),
`is
`widely used to provide information on chemical structures and
`physical forms.1 Theoretical calculations, especially density func-
`tional theory (DFT) and MøllerPlesset second-order perturba-
`tion theory (MP2) calculations, usually have been used to
`optimize the structures and predict the energies and frequencies
`of molecules or ions.29 Recently, Raman spectroscopy combined
`with theoretical calculations was found to be an excellent method
`for investigating the conformations of flexible molecules in liquid
`phase.1019 By comparing the experimental Raman spectrum of
`the liquid and the calculated Raman spectra of isolated molecules,
`the conformational equilibrium in the liquid can be explored.1319
`With this method, Ishiguro and co-workers studied several flexible
`molecules. They first found that two conformers coexist in the
`
`liquid of bis(trifluoromethanesulfonyl)imide (TFSI
`) and con-
`
`cluded that conformational equilibrium of TFSI
`in the liquid is
`established between the C1 and C2 conformers.13 They obtained
`similar results when they studied the ionic liquid 1-ethyl-3-

`
`).14
`methylimidazolium bis(fluorosulfonyl)imide
`(EMI
`FSI
`Thereafter, it was found that the Raman and IR spectra, over a
`wide frequency range, of liquid N,N-dimethylacrylamide (DMAA)
`and N,N-dimethylpropionamide (DMPA) could be satisfactorily
`explained in terms of planar cis and nonplanar staggered
`conformers.15 Very recently, Dobrowolaki and co-workers studied
`
`the liquid allyl acrylate with a similar method and found that all 10
`of the predicted conformers might coexist in the liquid.17
`Recently, temperature-dependent Raman spectroscopy has
`been employed to probe the conformational preferences of
`flexible organic molecules.10,1218,20 Lassegues and co-workers
`found that the C2 conformer is more stable than the C1 conformer
`
`) solution.10
`in bis(perfluoroethanesulfonyl)imide anion (BETI
`Durig and co-workers revealed that the cis conformer is more
`stable than the gauche conformer in both the gaseous and liquid
`phases of 3,3-difluorobutene.18 Moreover, studies have also
`indicated that vibrational spectroscopy, especially temperature-
`dependent vibrational spectroscopy, can be used as a tool for
`studying the hydrogen bonding of H-bond liquids.2022
`2-Phenylethylamine (PEA, C6H6CH2CH2NH2), which
`contains a rigid skeleton ring and a flexible ethylamine side chain,
`is one of the most important flexible organic molecules and the
`simplest member of a range of aromatic amine neurotransmitters.
`It has been found that PEA can cross the bloodbrain barrier and
`concentrate in the mammalian brain23,24 and can modulate the
`affective behaviors.25 To conduct basic research on PEA is
`valuable for the study of its biological functions in its native
`environment. Additionally, the exploration of the conformational
`
`Received:
`Revised:
`
`September 26, 2010
`March 1, 2011
`
`Published: March 18, 2011
`
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`preferences of PEA is helpful to understand the structures and
`properties of some other neurotransmitters, such as ampheta-
`mine, dopamine, serotonin, and histamine.2,5
`Conformations of PEA and other flexible molecules in the gas
`phase have been studied by many spectroscopic methods com-
`bined with theoretical calculations.27,26 As early as 1995, Godfrey
`and co-workers theoretically predicted that PEA has two types of
`stable conformations (two anti and three gauche conformations)
`that mainly differ in terms of the orientation of the amine group.
`Moreover, two of them can be characterized by millimeter-wave
`spectroscopy.2 Thereafter, Simons et al.3 and Alonso et al.4
`revealed that four PEA conformers coexist in the gas phase. These
`studies24 also indicated that, as an isolated molecule, gauche PEA,
`which can form an aromatic hydrogen bond (a weak NH 3
`3 π
`3
`interaction between the amine group and the aromatic ring), is
`more stable than anti conformers.4 Recently, Bar and co-workers
`explored the Raman spectral features of four conformers of PEA
`using the novel method of ionization-loss stimulated Raman
`spectroscopy (ILSRS)5 and found that the calculated Raman
`spectra of the PEA molecules agreed with the observed spectra
`very well.5 However, to our knowledge, no studies on conforma-
`tional equilibrium in liquid PEA have been performed.
`In this work, the conformational equilibrium and hydrogen
`bonding in liquid PEA were explored by Raman spectroscopy
`combined with theoretical calculations (DFT/MP2). To acquire
`more information on the conformational preferences and hydro-
`gen bonding in the liquid PEA, the temperature-dependent
`Raman spectra of PEA were also measured.
`
`2. METHODS
`
`2.1. Experimental Details. Liquid 2-phenylethylamine (PEA)
`(>98% pure) was purchased from Aladdin Reagent Company
`(Shanghai, China) and used as received.
`All Raman spectra of liquid PEA were recorded in the full
`spectral
`range with a confocal micro-Raman spectrometer
`(HORIBA Jobin Yvon HR800) and with a 532-nm solid-state
`laser as the excitation source. The exposure time during the
`measurements was 30 s. The power density was kept about 1 mW
`at the sample location, which is low enough to avoid heating
`effects. The temperature-dependent Raman spectra of liquid PEA
`were obtained while raising the temperature from 223 to 393 K,
`using a Linkam TMS 94 hot stage with a temperature stability of
`(0.1 K. The temperature was held at each selected value for at
`least 5 min. The freezing and boiling points of liquid PEA are ca.
`213 and 470 K, respectively; thus, PEA was in the liquid phase
`throughout all measurements.
`2.2. Computational Details. The geometry optimizations,
`one-dimensional potential energy scans (PESs), transition state
`searches, and harmonic frequency and Raman scattering activity
`calculations of PEA molecules were performed with the Gaussian
`03 package.27 Compared to the MP2 method, the B3LYP
`method is inexpensive, and its prediction of normal-mode
`frequencies are reliable.28,29 The harmonic frequencies of all
`PEA conformers were calculated at the B3LYP/aug-cc-pvdz
`level, and the geometries of all conformers were optimized at
`this theoretical level before the harmonic frequencies and Raman
`activities were calculated. The harmonic frequencies calculated
`with the basis set were scaled with two scaling factors that were
`successfully used in the similar systems before:30,31 0.959 for the
`CH and NH stretching vibrations (derived from the pre-
`viously reported scaling factor for valence AH stretching force
`
`Figure 1. Molecular structure of PEA.
`
`constants30) and 0.987 for all other vibrations. It can be seen that
`these scaling factors yielded good agreement between the
`observed and calculated frequencies for the PEA conformers.
`However, the MP2 method, which accounts for electron
`electron correlation, can obtain more reliable results for correla-
`tion energies than the B3LYP method.28 Therefore, the one-
`dimensional potential energy scans, transition state searches, and
`correlation energies (thermal energies and Gibbs energies) of
`PEA were predicted by MP2/6-311G(d,p).3 The calculations
`were performed for isolated molecules, without taking into
`account intermolecular interactions.
`The Raman intensities (Ii) of PEA can be derived from the
`calculated Raman scattering activities (Si) of the normal modes.
`The intensity of a Stokes Raman band (Ii) is proportional to its
`differential scattering cross section (∂σ/∂Ω)32,33
`Ii µ ½Dσ=Dي
`i
`The theoretical differential scattering cross section associated
`with normal mode Qi is given by33,34
`½Dσ=Dي
`¼ Dðν0 νiÞ4½1 expð hνic=kTފ1Si
`i
`where the constant D is the suitably chosen common normal-
`ization factor for all peak intensities; ν0 is the wavenumber of
`exciting laser radiation (in our calculations, we used ν0 =
`1, which corresponds to the laser radiation used);
`18796.0 cm
`νi is the theoretical wavenumber of normal mode Qi; h, c, k, and T
`are the Planck constant, speed of light, Boltzmann constant, and
`temperature in Kelvin (298.15 K), respectively; and Si is the
`theoretical Raman scattering activity of normal mode i (Å4
`1). For the plots of simulated Raman spectra, the harmonic
`amu
`frequencies were convoluted with pure Lorentzian lines with a
`1.
`full width at half-maximum (fwhm) of 10 cm
`
`3. RESULTS AND DISCUSSION
`
`3.1. Theoretical Calculations. Figure 1 shows the chemical
`structure of the PEA molecule with atom labeling. By changing
`the internal rotation angle of the ethyl group (C12C15), three
`types of conformations can be found, which include one anti and
`two gauche conformations. Several possible conformations,
`associated with the rotation of the phenyl ring, were ignored
`because of their high energy. Using starting structures proposed
`in previous works,25 one-dimensional potential energy scans of
`the anti and gauche conformers were performed by changing the
`NH2 internal rotation angle in 3.6° steps between 0° and 360° at
`the MP2/6-311G(d,p) level of theory. The potential energy
`curves as a function of the NH2 internal rotation angle are shown
`
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`Figure 2. (a) Nine possible conformations of PEA obtained by changing the ethyl group and the NH2 group. (bd) Potential energy curves of the (b)
`anti and (c,d) gauche conformers of PEA as a function of the NH2 internal rotation angle obtained at the MP2/6-311G(d,p) level.
`in Figure 2bd. It was found that each torsion energy potential
`curve has three stationary points. The nine different stationary
`points suggest that there are nine possible conformations. It can
`be concluded that all nine conformers are local minimum-energy
`conformations because of the absence of imaginary harmonic
`frequencies. The nine conformers are shown in Figure 2a.
`From Figure 2a, one can see that all nine conformations can be
`approximately described by two flexible internal coordinates: the
`C12C15 internal angle that specifies the position of the NH2
`group relative to the phenyl ring and the C15N internal rotation
`angle that specifies the orientation of the NH2 group. The
`position of the NH2 group is defined as anti (A, with the NH2
`group pointing away from the phenyl ring) or gauche (G, with the
`NH2 group pointing toward the phenyl ring). The nine con-
`0
`0
`0
`formers are labeled as A1, A1
`(A2), A3, G1, G2, G3, G1
`, G2
`, and
`0
`0
`. It was found that the conformer pairs A1 and A1
`(A2), G1
`G3
`0
`0
`0
`and G1
`, G2 and G2
`, and G3 and G3
`have the same energies and
`frequencies. Therefore, there are four degenerate conformers
`among the nine conformers. Thus, only five typical conformers
`(A1, A3, G1, G2, and G3) were considered in this report, which is
`consistent with previous reports.25
`In Figure 2bd, the barrier of A3 f A1 interconversion was
`1, and the A1 f A1
`0
`0
`(A2) and A1
`calculated to be ca. 1046 cm
`
`(A2) f A3 barriers were calculated as ca. 580 and 948 cm1,
`respectively. The G1 f G3, G1 f G2, and G3 f G2 barriers are
`
`01, respectively. The G10 f G3
`
`0 f
`ca. 868, 1185, and 951 cm
`, G1
`1,barriers are ca. 869, 1186, and 948 cm
`0
`
`0 f G20, and G3
`G2
`
`
`respectively. It is well-known that the flexible molecules can easily
`change from one conformation to another when the freedom
`barrier heights between the conformers are below 2.9 kcal/mol
`1).35 Because the interconversion barriers between
`(ca. 1000 cm
`1,
`the PEA conformers are slightly higher or lower than 1000 cm
`
`the anti and gauche conformers are expected to interchange easily.
`0
`0
`In contrast, the A3 f G1 (G1
`) and A3 f G2 (G2
`) barriers are
`1, and the A3 f G3 (G3
`1.) barriers is ca. 1385 cm
`0
`
`ca. 1470 cm
`0 f G10
`
`
`
`0), A1 f G2 (A10 f G2
`The barriers of A1 f G1 (A1
`),
`1,) are ca. 1282, 949, and 1282 cm
`0
`0
`) f G3 (G3
`and A1 (A1
`
`0
`0 f G1) and A1 f G20(A1
`
`respectively. The barriers of A1 f G1
`
`
`0 f G2) are ca. 1349 cm(A1 1 [obtained by transition state
`
`searches that were conducted employing the synchronous transit-
`guided quasi-Newton (STQN) QST2 method3638]. This in-
`dicates that the interchange between anti and gauche conformers
`should overcome a higher barrier.
`A selection of geometrical parameters and the corresponding
`relative energies (values of the thermal energy, ΔE, and Gibbs
`energy, ΔG, relative to the G1 conformer) of the five conformers
`of PEA are shown in Table 1, which were obtained from MP2/6-
`311G(d,p) calculations. From the data in Table 1, one can also see
`that the differences among the conformers of PEA molecules
`mainly exist in the terminal flexible side chain. The anti and
`gauche conformers essentially differ in the internal rotation angle
`of the ethyl group (namely, the C4C12C15H16 dihedral
`angle). The three gauche conformers mainly differ in the orienta-
`tion of the NH2 group (namely, the C12C15N18H19
`dihedral angle) from the anti conformers. On the other hand,
`Table 1 shows that G1 and G2, which have a NH 3
`3 π
`3
`hydrogen bond, are much more stable than G3. In Table 1, the
`order of the thermal energy of the five conformers is G1 < G2 <
`A3 < A1 < G3. G1 is the most stable structure among the five
`isolated conformers, and the stability of the five conformers is in
`line with previous results.2,3,5 In addition, the differences in energy
`between the conformers are less than 1.6 kcal/mol (see Table 1).
`These indicate that all five conformers can coexist in conforma-
`tional equilibrium in liquid PEA.
`
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`Table 1. Geometrical Parameters of the Five Conformers and Their Corresponding Relative Energiesa
`
`A1
`
`1.0163
`1.0158
`
`107.50
`107.05
`105.83
`
`32.90
`57.03
`67.04
`
`A3
`
`1.0163
`1.0163
`
`107.31
`106.57
`105.72
`
`33.18
`57.84
`57.46
`
`five predicted conformers
`
`G1
`
`G2
`
`bond lengths (Å)
`1.0155
`1.0165
`
`bond angles (deg)
`108.18
`107.51
`106.19
`
`dihedral angles (deg)
`36.50
`179.06
`174.89
`
`1.0165
`1.0164
`
`107.49
`107.07
`105.80
`
`41.22
`178.16
`56.62
`
`G3
`
`1.0165
`1.0155
`
`106.98
`107.50
`106.08
`
`29.78
`165.28
`72.89
`
`N18H19
`N18H20
`
`H13C12H14
`H16C15H17
`H19N18H20
`
`C3C4C12H13
`C4C12C15H16
`C12C15N18H19
`
`1)
`relative energiesb (kcal 3 mol
`ΔE
`1.58
`0.05
`0.00
`1.06
`1.34
`ΔG
`1.35
`0.08
`0.00
`0.62
`0.80
`a Labeling of atoms is provided in Figure 1. The data were predicted at the MP2/6-311G(d,p) level. b Thermal energies, ΔE, and Gibbs energies, ΔG,
`relative to the G1 conformer.
`
`structures of the calculated conformers, we juxtaposed the
`selected (550800 and 12501500 cm
`1 regions) experimental
`Raman spectra of liquid PEA at 303 K and the calculated Raman
`spectra of all five conformers (A1, A3, G1, G2, and G3) (see
`Figure 4).
`Figure 4 shows the experimental Raman spectra and the
`theoretical predicted spectra of the five conformers in the
`selected ranges of 550800 and 12501500 cm
`1. Wavenum-
`bers of all of the experimental Raman bands of the liquid and the
`calculated Raman bands of the five conformers in the selected
`region are also listed in Table 2, together with rough assignments
`of the bands. The assignments were done by the assistance of the
`theoretical predictions, and some of them are based on the
`previous studies.5,3941
`
`In the range of 550600 cm1, two weak bands at 568 and
`1 can be seen in the experimental Raman spectrum. The
`592 cm
`1 can be seen in the predicted spectra of
`band located at 568 cm
`1 corresponds
`only gauche conformers, and the band at 592 cm
`only to anti conformers. By the assistance of the theoretical
`1 can be attributed to CH out-
`predictions, the band at 568 cm
`of-plane bending and C15H2 rocking of the gauche conformers,
`1 can be attributed to ring phenyl in-
`and the band at 592 cm
`plane deformations of the anti conformers. Both of these bands
`appear in the measured Raman spectra, suggesting that at least
`one of the gauche conformers and one of the anti conformers
`exist in the liquid PEA.
`In the range of 700800 cm
`1, the bands at ca.752 and
`1 can be observed in the experimental spectrum of liquid
`762 cm
`1 is assigned to CH out-of-plane
`PEA. The band at ca. 752 cm
`bending. It appears in the Raman spectra of all five predicted
`1, which mainly
`conformers. However, the band at ca. 762 cm
`corresponds to C15H2 rocking and CH out-of-plane bending,
`evidently exists only in the spectra of gauche conformers, which
`confirms that at least one of the gauche conformers exists in
`liquid PEA.
`In the range of 12501500 cm
`1, conformers of PEA can be
`distinguished by the characteristic bands of their Raman spectra.
`
`Figure 3. Measured Raman spectrum of liquid PEA (at 303 K) in the
`regions of 1001800 and 27003500 cm
`1.
`
`3.2. Conformational Equilibrium in Liquid PEA. Figure 3
`shows the Raman spectrum of liquid PEA at 303 K in the regions
`1001800 and 27003500 cm
`1. Comparisons of the observed
`and calculated frequencies are presented in Table S1 of the
`Supporting Information. The scaling factor 0.987 was applied to
`the calculated frequencies in the range of 1001800 cm
`1, and
`0.959 was applied in the range of 27003500 cm
`1. Comparison
`of the observed and calculated Raman frequencies indicates that
`different conformers of PEA are present in the liquid.
`Previous experimental and computational results showed that
`at least four conformers coexist in the conformational equilibri-
`um of gas-phase PEA.35 Comparison (see Table S1 of the
`Supporting Information) indicates that different conformers
`in the 550800 and
`have different characteristic bands
`12501500 cm
`1 regions, as marked by asterisks in Figure 3.
`To find some vibrational modes related to specific molecular
`
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`Figure 4. Comparison of the experimental Raman spectra of liquid PEA with the calculated Raman spectra of the five PEA conformers in the regions of
`(a) 550800 and (b) 12501500 cm
`1.
`
`Table 2. Wavenumbers of All Experimental Raman Bands in the
`Selected Regions (550800 and 12501500 cm
`1) and Cor-
`responding Predicted (B3LYP/aug-cc-pvdz) Raman Bands of
`the Five Conformers, with Rough Assignments of the Bandsa
`
`five predicted conformers
`
`1) A1
`exp (cm
`
`597
`
`568 (w)
`592 (w)
`
`621 (m)
`621
`752 (m)
`750
`762 (m)
`
`G1
`
`A3
`
`568
`592
`622
`622
`749
`748
`
`764
`
`G2
`
`G3
`
`568
`
`
`621
`745
`754
`
`568
`
`
`621
`747
`760
`
`assignment
`β(CH) op þ γ(C15H2)b
`ring def ipb
`ring def ipb,c
`β(CH) opb,c
`γ(C15H2) þ β(CH) opb
`
`1291 (w)
`1326 (w)
`
`
`
`1283
`
`1290
`
`1291
`1320
`1323 1327
`
`ω(C12,15H2)b,c
`τ(C12,15H2)b
`τ(C15H2,NH2) þ
`ω(C12H2)b,c
`β(CH) ip þ ω(C12H2)b,c
`1334 1336 1331 1335
`1336 (w)
`
`
`
`1340 (w)
`1342 τ(C15H2) þ ω(C12H2)b
`1354 (w)
`1353
`
`
`ω(C12,15H2)b,c
`
`τ(C15H2) þ τ(NH2)b
`1364 (w)
`
`1359
`1380 (w)
`
`1382
`1380 ω(C15H2) þ τ(NH2)b,c
`1386
`
`
`
`ω(C12,15H2) þ τ(NH2)b,c
`1393 (w)
`
`
`1404 (w)
`
`
`1441 (m)
`
`1441
`
`δ(C12H2)b,c
`1446 (m)
`1446
`1444 1444 δ(C12,15H2)b,c
`1454
`
`1452
`δ(C12,15H2)b,c
`1454 (w)
`
`1472 (w)
`1472
`1468 δ(C15H2)b
`1477
`
`
`
`δ(C12,15H2)b
`1480 (w)
`a Abbreviations used: m, medium; w, weak; β, bending; γ, rocking; τ,
`twisting; ω, wagging; δ, scissoring; op, out of plane; ip, in plane; def,
`deformation. b From the DFT calculation results in this work. c From refs
`5, 39, 40, and 41.
`
`1, which is assigned to C12H2
`The band located at 1441 cm
`scissoring, is related only to G1 conformer. Furthermore, G1 is
`the lowest-energy conformer among the five. Thus, the G1
`1,
`conformer should exist in the liquid. The band at 1354 cm
`which is assigned to the C12H2 and C15H2 wagging, can be
`explained by only the A3 conformer. Additionally, A3 is the
`lowest-energy conformer among the anti conformers. This
`indicates that A3 conformer cannot be excluded in liquid PEA.
`1 is attributed to the
`The weak band observed at 1291 cm
`C12H2 and C15H2 twisting, which is related to conformer A1 or
`1 assigned to C15H2
`G3. Furthermore, the band at 1340 cm
`twisting and C12H2 wagging is related to only the G3 conformer.
`Therefore, G3 exists in liquid PEA. As A1 is lower in energy than
`G3, A1 can exist in the conformational equilibrium of liquid-
`1, which is related to the
`phase PEA. The weak band at 1364 cm
`C15H2 and NH2 twisting modes, can be found only in the G2
`conformer, which indicates that the G2 conformer is also present
`in liquid PEA. These results confirm that all five conformers
`coexist in the conformational equilibrium of liquid PEA.
`3.3. Hydrogen Bonding in Liquid PEA. Two kinds of
`hydrogen bonds (i.e., NH 3
`3 π and NH 3
`3
`3
`3 N hydrogen
`bond) can form in liquid PEA. Wavenumbers of the NH2
`symmetric stretching band of liquid PEA at 303 K, of isolated
`PEA in the gas phase,5 and of the five predicted conformers are
`listed in Table 3.
`In Table 3, the Raman spectra of the isolated PEA molecules in
`the gas phase show that the Raman band of the nonbonded or
`NH 3
`3 π hydrogen-bonded NH2 symmetric stretch can be
`3
`observed in the range of 33403350 cm
`1. The NH2 symmetric
`1) is red-
`stretching mode of gauche conformers (at ca. 3345 cm
`1 compared to that of anti conformers (at
`shifted by about 5 cm
`1), because of the presence of weak intramolecular
`ca. 3350 cm
`NH 3
`3 π hydrogen bonds in the former.5 However, no
`3
`obvious band in the range of 33403350 cm
`1 can be observed
`in the experimental Raman spectrum of liquid PEA. Note that
`i.e., NH 3
`3 π and
`3
`only two kinds of hydrogen bonds,
`
`3064
`
`dx.doi.org/10.1021/jp109194v |J. Phys. Chem. A 2011, 115, 3060–3067
`
`The Journal of Physical Chemistry A
`
`ARTICLE
`
`Merck Exhibit 2236, Page 5
`Mylan v. Merck, IPR2020-00040
`
`

`

`NH 3
`3 N hydrogen bonds, can be formed in the liquid PEA.
`3
`This suggests that at least one of the hydrogen atoms of the
`amine group is involved in intermolecular NH 3
`3
`3 N hydrogen
`bonding in liquid PEA.
`1, which
`Furthermore, the intense band at ca. 3318 cm
`corresponds to the NH2 symmetric stretching mode, is observed
`in the experimental Raman spectra of liquid PEA. Comparing
`the experimental Raman spectrum of liquid PEA with that of
`the isolated PEA molecules5 and the theoretical predictions, the
`NH2 symmetric stretching mode band is red-shifted by about
`1. Obviously, the large red shift cannot result from the
`30 cm
`contribution of the intramolecular NH 3
`3 π hydrogen bond. A
`3
`previous study revealed that strong hydrogen bonding can bring
`about a red shift in the stretching frequency from several tens to
`a few hundred wavenumbers.28 Thus, the strong intermolecular
`NH 3
`3
`3 N hydrogen bond could contribute to the large red shift
`in the Raman spectrum of liquid PEA. It also shows evidence that
`the amine group is involved in the intermolecular hydrogen bond.
`As mentioned above, the NH2 symmetric stretching mode of
`non-hydrogen-bonded or NH 3
`3 π hydrogen-bonded PEA
`3
`would appear in the range of 33403350 cm
`1.5 However, no
`obvious band can be observed in that region in the measured
`Raman spectrum of liquid PEA. Therefore, G1 and G2, which
`
`1) of the NH2
`Table 3. Comparison of Wavenumbers (cm
`Symmetric Stretch from the Experimental Raman Spectrum of
`Liquid PEA (at 303 K), the Experimental Results in the Gas
`Phase from Ref 5, and the Calculated Raman Spectra of Five
`PEA Conformers
`
`A1
`A3
`G1
`G2
`G3
`
`this work
`
`3318
`
`ref 5
`
`3350
`3349
`3345
`3344
`
`calc
`
`3340
`3337
`3342
`3336
`3343
`
`contain the NH 3
`3 π hydrogen bond, might not be dominant in
`3
`the liquid. Note that G3 has the highest energy among the five
`conformers. This implies that anti conformers might be dominant
`in the liquid, which is different from the case for PEA in a
`supersonic jet. Because the PEA molecules in a supersonic jet
`are “isolated”,5 G1 and G2 can easily form intramolecular N
`3 π hydrogen bonds. In contrast,
`H 3
`3
`in the liquid, the PEA
`molecules can form intermolecular NH 3
`3
`3 N hydrogen bonds,
`and the anti conformers have less steric hindrance. Therefore, the
`anti conformers would be favorable in liquid PEA. This result is
`consistent with those of previous studies on other flexible liquid
`molecules.10,13,15,18
`3.4. Temperature Dependence of Raman Spectra. To get
`more information on the conformational preferences and hydro-
`gen bonding in liquid PEA, the temperature-dependent Raman
`spectra of liquid PEA were measured from 223 to 393 K. The
`temperature-dependent Raman spectra
`in the ranges of
`1001800 and 27003500 cm
`1 are shown in Figure S1 of
`the Supporting Information. It can be seen that only the features
`in the two narrow ranges 700820 and 30003400 cm
`1 show
`significant changes. The spectra in these two regions are enlarged
`and shown in Figure 5. In Figure 5a, the Raman spectra exhibit a
`significant temperature dependence, indicating that the equilib-
`rium in liquid PEA changes at different temperature. It can be
`1, which is
`seen that the relative intensity of the band at 752 cm
`assigned to CH out-of-plane bending, decreases when the
`temperature is increased. However, the relative intensity of the
`1, which is related to CH2 rocking and CH
`band at 762 cm
`out-of-plane bending and can be seen only in the gauche
`increases when the temperature is raised. This
`conformers,
`implies that the concentration of gauche conformers would
`increase when the temperature of liquid PEA is increased. These
`results also confirm that anti conformers would be more favor-
`able in liquid PEA at room temperature.
`Additionally, it was found that the wavenumbers of the NH2
`1 whenstretching mode are blue-shifted from 3312 to 3322 cm
`
`the temperature is increased from 223 to 393 K (see Figure 5b).
`
`Figure 5. Temperature-dependent Raman spectra of liquid PEA in the regions of (a) 700820 and (b) 30003400 cm1 obtained as the temperature
`
`
`raised from 223 to 393 K.
`
`3065
`
`dx.doi.org/10.1021/jp109194v |J. Phys. Chem. A 2011, 115, 3060–3067
`
`The Journal of Physical Chemistry A
`
`ARTICLE
`
`Merck Exhibit 2236, Page 6
`Mylan v. Merck, IPR2020-00040
`
`

`

`As mentioned above, the NH2 symmetric stretching mode of
`non-hydrogen-bonded and NH 3
`3 π hydrogen-bonded PEA
`3
`is located in the range of 33403350 cm
`1. Hence, those bands
`are mainly related to intermolecular NH 3
`3
`3 N hydrogen
`bonding. When the temperature is increased, the intermolecular
`hydrogen bonds among molecules break and weaken.21 Thus,
`the red shift of bands of the NH2 stretching mode become
`smaller. Therefore, those bands show a blue shift and are close to
`the wavenumber of the free NH stretch band (ca. 3350 cm
`1)5
`when the temperature is increased. This also shows evidence for
`the existence and the important role of intermolecular hydrogen
`bonds in liquid PEA.
`
`4. CONCLUSIONS
`
`Using a potential energy scan achieved by changing the ethyl
`and NH2 internal rotation angles with MP2 theoretical methods,
`it has been predicted that PEA has nine stable conformers,
`including four degenerate conformers. Comparison of the ex-
`perimental Raman spectrum of liquid PEA at 303 K and the
`calculated Raman spectra of the five conformers indicates that all
`five conformers can coexist in the conformational equilibrium of
`liquid PEA. The large red shift of the NH2 stretching mode
`indicates that the intermolecular NH 3
`3
`3 N hydrogen bonds
`play an important role in liquid PEA.
`The temperature-dependent Raman spectra show that the
`1 band, which corresponds to
`relative intensity of the 762 cm
`CH2 rocking and CH out-of-plane bending of the gauche
`conformers, increases with increasing temperature. This indi-
`cates that the anti conformers are more favorable than the
`gauche one in liquid PEA at room temperature. When the
`temperature of
`liquid PEA is increased, the bonded NH2
`stretching band is blue-shifted, which implies that the intermo-
`lecular hydrogen bond is weakened. This also provides evidence
`for the existence and the important role of
`intermolecular
`hydrogen bonds in liquid PEA.
`
`’ ASSOCIATED CONTENT
`bS
`Supporting Information. Comparison between the ob-
`served and calculated frequencies (Table S1) and Raman spectra
`measured at different temperatures (Figure S1). This material is
`available free of charge via the Internet at http://pubs.acs.org.
`
`’ AUTHOR INFORMATION
`
`Corresponding Author
`*Phone: 86-20-85217070. Fax: 86-20-85216052. E-mail: yjhu@
`scnu.edu.cn..
`
`’ ACKNOWLEDGMENT
`
`supported by NSFC (Nos. 20973067,
`This work was
`11079020) and Guangdong-NSF (No. 7005823) grants, the
`scientific research foundation for returned overseas Chinese
`scholars, State Education Ministry, the foundation for introduc-
`tion of talents by the universities in Guangdong Province, and the
`project under scientific and technological planning by Guangz-
`hou City. The authors appreciate Prof. Yulong Liu, Dr. Ke Zhu,
`and Mr. Liang Jing of the Chinese Academy of Sciences.
`
`’ REFERENCES
`
`J. Phys. Chem. A 2007,
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`(1) Smith, W. E.; Dent, G. Modern Raman Spectroscopy: A Practical
`Approach: John Wiley & Sons: Chichester, U.K., 2005.
`(2) Godfrey, P. D.; Hatherley, L. D.; Brown, R. D. J. Am. Chem. Soc.
`1995, 117, 8204–8210.
`(3) Dickinson, J. A.; Hockridge, M. R.; Kroemer, R. T.; Robertson,
`E. G.; Simons, J. P.; McCombie, J.; Walker, M. J. Am. Chem. Soc. 1998,
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`(4) Lopez, J. C.; Cortijo, V.; Blanco, S.; Alonso, J. L. Phys. Chem.
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