The article information
 Zhenlin Zhang, Wenlou Wang, Shilin Liu, Dongming Chen
 章振林, 王文楼, 刘世林, 陈东明
 Experimental and Density Functional Theory Calculation Studies on Raman and Infrared Spectra of 1, 1'Binaphthyl2, 2'diamine
 1, 1'联萘2, 2'二胺拉曼和红外光谱的实验和DFT计算研究
 Chinese Journal of Chemical Physics, 2017, 30(1): 715
 化学物理学报, 2017, 30(1): 715
 http://dx.doi.org/10.1063/16740068/30/cjcp1606118

Article history
 Received on: June 1, 2016
 Accepted on: June 3, 2016
b. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
The axially chiral binaphthyl compounds play a crucial role in constructing new stable chiral structures as well as in developing effective asymmetric organocatalysts [118]. Various spectroscopic methods have been used to explore the structures and properties of the binaphthyl compounds, especially the 1, 1'binaphthyl2, 2'diol, in the past several decades [1931]. For example, Nogueira et al. measured the surfaceenhanced Raman spectrum (SERS) of 1, 1'binaphthyl2, 2'diol on silver colloids and proposed the empirical assignments for the observed SERS bands [19]. In Ref.[20], the vibrational circular dichroism (VCD) spectrum of 1, 1'binaphthyl2, 2'diol was studied and density functional theory (DFT) calculations were carried out to assign the VCD bands. With ultralowfrequency Raman technique, Chang et al. studied the polymorphic transformation of crystalline binaphthyls [21]. Li et al. studied the normal and UV nearresonant Raman (UVRR) spectra of 1, 1'binaphthyl2, 2'diol in basic solution and assigned the observed Raman bands on the basis of the DFTcalculations [22]. Vibrational spectroscopy and DFT calculations have also been extensively used to investigate the chiral conformational stability of the binaphthyl compounds [2325]. The solutions of 1, 1'binaphthyl2, 2'diol have also been studied by the sumfrequency generation (SFG) spectroscopies [2630]. Recently, Liégeois have conducted a comprehensive theoretic study on the polarized Raman and vibrational Raman optical activities (VROA) of a series of 2, 2'substituted binaphthyl compounds [31].
Among various binaphthyl compounds, the 1, 1'binaphthyl2, 2'diamine (BINAM) derivatives have been found highly useful in developing new asymmetric catalysts [718]. For example, Guillena et al. have synthesized a series of BINAMprolinamides and used them as catalysts in several organic reactions, such as the direct aldol condensation between aldehydes and aliphatic ketones [812]. Starting from BINAM, Duan et al. have obtained a novel axially chiral Rhcomplex and applied it in the Rhcatalyzed enantioselective hydrosilylation of methyl ketone [13]. By using BINAMderived phosphoric acids, Nelson et al. recently developed a chiral anion phasetransfer stretagy to achieve highly enantioselective αamination of carbonyl compounds [15]. Barbas group has discovered a highly efficient organocatalytic domino approach for the direct construction of bispirooxindole derivatives with excellent stereocontrol by employing an organocatalyst containing a BINAM skeleton [17].
On the other hand, despite of its significance in asymmetric catalysis, the spectroscopic properties of BINAM have not been well studied. To the best of our knowledge, the experimental and theoretical study of Raman and IR spectra of BINAM has never been reported. Therefore, the present study was conducted with the intention to provide a detailed vibrational analysis for BINAM by conjunct investigation of IR/Raman experiments and DFT calculations. The assignments of the observed IR and Raman bands have been proposed based on the local vibrations of substituted 2naphthylamine. The UVRR spectrum of BINAM was also measured and its resonant enhancement pattern was discussed and used to analyze the possible distortion of the excited state structure.
II. Experimental and Computational Methods1, 1'binaphthyl2, 2'diamine (>99%) was purchased from Alfa Aesar and was used without further purification. UVVis absorption spectra were measured with a 5 mm lightpath quartz cell at room temperature using Shimadzu UV2401PC spectrometer. Raman spectra were recorded on a LABRAMHR 800 microRaman spectrometer equipped with an aircooled CCD detector, a notch filter, and a 600 grooves/mm grating for the visible excitation and a 2400 grooves/mm grating for UV. The 325 nm line of a HeCd laser and the 488.0 nm line of an Ar ion laser were used as the excitation sources with the power 3.5 and 5 mW, respectively, on the sample. The collection time of the CCD detector was 40 s for Raman measurements. In order to minimize any damage of the sample due to the prolonged exposure to laser radiation, the BINAM solid sample was mixed with KBr powder in a weight ratio of 1:10 and pressed into a disc of 1 cm diameter, which was placed on a rotating holder during Raman measurements. IR spectrum of BINAM was measured on a Nicolet 6700 FTIR spectrometer as KBr pallet.
DFT calculations were carried out using Becke's threeparameter hybrid functional (referred as B3LYP) [32, 33], which has been proven to be suitable for studying the structures and properties of binaphthyl systems [22, 25, 2931]. In order to reduce computational cost, initial searching of steady structure of the studied molecule was carried out by geometry optimization with relative small basis sets, 631G, without any symmetry constraint. The obtained structures were then used for the final optimization using 6311++G(d, p) basis sets with suitable symmetry (C_{2} point group) constraint. Analytic frequency calculations (using B3LYP/6311++G(d, p)) at the optimized structure were done to confirm the optimized structures to be an energy minimum and to obtain the theoretical vibrational spectra. Due to the neglect of the anharmonicity and the incomplete basis sets, the DFT calculations tend to slightly overestimate the vibrational frequencies. These systematic discrepancies between the computed and experimental frequencies can be corrected by scaling the calculated frequencies with a single factor of 0.98, which has been found to be appropriate for the planar conjugate systems [34, 35]. Assignment of individual vibrational frequency was carried out by inspecting the calculated Cartesian displacements of the corresponding normal mode. Electronic absorption spectrum of BINAM was theoretically studied with TDDFT calculations [3638]. The simulation of UVresonance Raman spectrum was carried out with frequencydependent polarizability using coupledperturbation theory (CPHF). All DFT/TDDFT calculations were performed with the Gaussian 09 program suite [39].
III. Results and discussion A. Groundstate geometries and UVvisible absorption spectrumTable I lists the bond lengths, bond angles and dihedral angles of BINAM optimized with B3LYP/6311++G(d, p). Figure 1 shows the structural sketch and atomic labels of BINAM used in this work. B3LYP/6311++G(d, p) calculations indicate that the two naphthyl rings of BINAM molecule are almost perpendicular with each other. The dihedral angle between the two naphthyl groups is about 91.4°. The C1C2, C2C3, and C9C10 distances of BINAM are calculated to be 1.394, 1.425, and 1.432 Å, respectively, while the C1C9 distance is found to be 1.431 Å. The CN bond length of BINAM is 1.398 Å.
Figure 2(a) displays the UVVis absorption spectrum of RBINAM dissolved in CH_{2}Cl_{2}, which shows a quite broad peak with the absorption maximum at 348 nm. Figure 2(b) gives the theoretical spectrum of BINAM calculated with TDB3LYP/6311++G(d, p), where a bandwidth of 2600 cm^{1} (0.322 eV) was utilized for each electronic transition in the spectral simulation. Table II tabulates the calculated orbital energies and symmetries of frontier molecular orbitals of BINAM, and Table III lists the calculated excitation energies and oscillator strengths of BINAM calculated with TDB3LYP/6311++G(d, p). Table III also gives the weights of major configurations for each excited state. According to the TDB3LYP calculations, the nearUV absorption of BINAM are contributed from the transitions from the ground state to 1^{1}B, 1^{1}A, 2^{1}B, and 2^{1}A states, with 1^{1}B, 2^{1}B, and 2^{1}A being of relatively large oscillator strengths. As shown in Table III, the transition energies of these four states are quite close with each other, while they are well separated from higher excited states (3^{1}B, etc.). Thus the observed broad 348 nm peak in Fig. 2(a) can be attributed to the overlap of the 1^{1}B, 2^{1}B, and 2^{1}A states.
B. Vibrational spectra 1. Infrared absoption spectrumBINAM has 38 atoms and 108 degrees of internal freedom, which, according to the C_{2} symmetry, can be classified as Γ=55A+53B. According to the vibrational selection rules, all of these modes are active for both IR and Raman transitions. As the two naphthyl rings of BINAM are nearly perpendicular to each other, most vibrational modes of BINAM can be considered as the inphase and outofphase combinations of the corresponding local vibrations of two isolated naphthylamine groups. The molecular vibrations of naphthalene have been studied by Scherer [40], but it is not very useful in helping our vibrational assignments since the substituents in BINAM significantly alter the mode compositions. Therefore, we conducted a supplementary frequency calculation on 1chloro2naphthylamine molecule and used the corresponding modes to describe and classify the normal vibrations of BINAM. In this way, the inphase or outofphase coupling (so called vibrational exciton coupling) of the corresponding local vibrations of two naphthylamine groups generates A and B symmetry blocks of normal modes of BINAM. The calculated atomic Cartesian displacements of 1chloro2naphthylamine are shown in Figs. S1 and S2 in the supplementary materials. Detailed vibrational assignments of BINAM will be discussed in the following parts of the article.
Figure 3 compares the experimental and theoretical IR spectra of BINAM, where the theoretical IR spectrum has been simulated from B3LYPcalculated harmonic frequencies (scaled with 0.98) and intensities. By comparing with the DFTcalculated band positions and intensities, the observed IR bands of BINAM are assigned as listed in Tables IV and Table V. In the Tables, the mode numbers are according to those of isolated 1chloro2naphthylamine molecule, and the single quotation marks are added after the mode numbers for the B symmetry modes so as to distinct them from the A modes. It is noticed that, while both A and B modes are IR active, B3LYP calculations indicate that most of the strong IR absorptions of BINAM in the high frequency region (9501600 cm^{1}) are due to the inplane naphthyl modes belong to the B symmetry block.
The vibrations in the region 12001650 cm^{1} are mainly due to the C=C stretches of naphthyl rings, and some of which are coupled with the NH_{2} scissoring vibrations. The strongest absorption was measured at 1616 cm^{1}, which is rather broad and can be decomposed into a main band at 1616 cm^{1} and a shoulder band at 1605 cm^{1}. DFTcalculations show a very strong IR band at 1624 cm^{1} and a shoulder band at 1615 cm^{1}. However, according to calculation the 1624 cm^{1} band itself is an overlapped band by two normal modes which can be assigned to v_{7} and v_{7} (the C=C stretches of the naphthyl rings), both of which are severely coupled with the NH_{2} scissoring vibrations. The shoulder band at 1605 cm^{1} can be assigned to v_{8}, which is calculated at 1615 cm^{1} and is also severely coupled with the NH_{2} scissoring. Other predominant IR absorptions in 12001650 cm^{1} region were measured at 1506, 1468, 1426, 1383, 1348, 1289, 1249, and 1208 cm^{1}. They are thought to correspond to the calculated bands at 1516/1513 (v_{10}/v_{10}'), 1468(v_{11}'), 1431(v_{12}'), 1377(v_{13}'), 1346(v_{15}'), 1283 (overlapped by v_{16} and v_{16}'), 1247(v_{17}'), and 1211 cm^{1}(v_{18}'), respectively.
In the region 9501200 cm^{1}, IR bands were observed at 1147, 1129, 1112, 1022, and 964 cm^{1} and the corresponding absorptions are calculated at 1152 (v_{20}'), 1110 (v_{21}), 1099 (NH_{2} rocking), 1026 (v_{22}'), and 965 cm^{1} (v_{23}'+NH_{2} rocking), respectively, with middle or weak intensities.
In 700950 cm^{1} region, IR absorptions were observed at 921, 869, 815, 782, and 761 cm^{1} and calculated at 920 (v24'), 856 (γ_{4})/855 (γ_{4}), 810 (γ_5/γ_{5}), 775 (γ_{6}), and 745/743 cm^{1} (γ_{7}/γ_{7}), respectively. According to the calculation, most of them are due to the outofplane wagging of the hydrogen atoms on the naphthyl rings. In the 400700 cm^{1} region, our B3LYPcalculation predicted three strong IR bands at 545 (γ_{10}+NH_{2} wagging), 511 (NH_{2} wagging+γ_{8}), and 490 cm^{1} (v_{29}+NH_{2} wagging) which can be assigned to the inplane or outofplane skeletal deformation of the naphthyl rings together with NH_{2} wagging). Experimentally, only weak absorptions were observed probably because of the interference of broad background in the lowfrequency region.
2. Normal Raman spectrum excited at 488.0 nmFigure 4(a) displays the normal Raman spectrum of BINAM in solid discs with 488.0 nm excitation, while the calculated nonresonant Raman is shown in Fig. 4(b). The experimentally observed and DFTcalculated frequencies and assignments of Raman bands of BINAM are listed in Tables IV and Table V.
In the nonresonance Raman spectrum of BINAM (Fig. 4(a)), strong Raman bands were observed at 1618, 1573, 1476, 1439, 1381, 1358, 1024, 852, 668, 577, and 528 cm^{1}. As shown in Tables IV and Table V, most of the vibrations of BINAM in the 9001650 cm^{1} region are due to the naphthyl inplane CC stretching and CH bending. In the 15501650 cm^{1} region, three bands were observed at 1618, 1594, and 1573 cm^{1}, respectively, in the normal Raman spectrum of BINAM. The 1573 and 1618 cm^{1} bands of BINAM are much stronger than the 1594 cm^{1} band. Theoretically, B3LYP/6311++G(d, p) calculations give rise to three bands at 1573, 1602, and 1624 cm^{1}, respectively, with similar relative intensities to the experiment. According to our DFT calculations, the strong 1573 cm^{1} band can be attributed to v_{9}, the stretching of C2C3/C6C7/C9C10 bonds of the naphthyl rings. On the other hand, the B3LYP/6311++G(d, p) calculation manifests that the bands at 1602 and 1624 cm^{1} are associated mainly with the naphthyl C3C4/C1C2/C5C6 stretching but they also contain significant contributions from NH_{2} scissoring. Similar to that observed in IR spectrum, the 1624 cm^{1} Raman band in Fig. 4(b) also comes from two overlapped normal modes, v_{7} and v_{7}. Two strong Raman bands were observed at 1476 and 1439 cm^{1} (Fig. 4(a)), which are thought corresponding to the calculated bands at 1474 (v_{11}) and 1434 cm^{1} (v_{12}). Based on our DFT calculations, these two bands are assigned to the CC stretching of naphthyl ring, and both of them also involves significant contributions from inplane CCH bending.
With 488 nm excitation, the strongest Raman band of BINAM was observed at 1381 cm^{1}, and also a strong Raman band at 1358 cm^{1} was observed at the lowfrequency side of the 1381 cm^{1}band. B3LYP/6311++G(d, p) calculation of BINAM predicts a very strong Raman at 1368 cm^{1} (v_{14}) and two shoulder bands at 1361 (v14') and 1351 cm^{1} (v_{15}), respectively, at lowfrequency side of the 1368 cm^{1} band. As the v_{15} mode (1351 cm^{1}) is calculated much weak in Raman intensity, we assign the observed 1381 and 1358 cm^{1} bands to v_{14} and v_{14}', both of which involves the inplane stretching of C9C10/C5C6/C8C9 bonds. The weak or shoulder bands at 1289, 1276, 1215, 1208, 1158, and 1146 cm^{1} are assigned to the v_{16}, v_{17}, v_{18}, v_{23}, v19'/v_{19}, and v_{20}'/v_{20} modes. Most of these modes are due to the inplane CCH bending of the naphthyl groups, with exception of v_{23} that is dominated with C1C1 stretching between two naphthyl groups. The strong Raman band at 1024 cm^{1} is calculated at 1026 cm^{1} (v_{22}). This mode also involves inplane CCH bending, but it contains a significant contribution of inplane deformation of the naphthyl rings.
Our DFT calculations manifest that the nonresonance Raman band in the 400950 cm^{1} region is mainly due to the inplane deformations of naphthyl ring. The observed weak Raman band at 960 cm^{1} was calculated at 945 cm^{1} and is assigned to inplane deformation v_{24} mixed slightly with outofplane deformation γ_{3}. The observed 852 cm^{1} band of BINAM is assigned to v_{25}, a naphthyl inplane deformation mode involving largely the changes of the C3C4C10/C6C7C8/C5C6C7 bondangles. DFT calculations predict this band to appear at 844 cm^{1}, with a pretty strong Raman intensity. Two weak bands at 794 and 774 cm^{1} were detected in the normal Raman spectrum. Based on matching of calculated and observed frequencies and intensities, we assign them to the calculated bands at 787 and 768 cm^{1}. According to the DFTcalculations, these two modes belong to B symmetry and correspond to the inplane skeleton deformations mixed with the CH outofplane wagging of the naphthyl rings. DFT calculation for BINAM predicts a rather strong Raman band at 665 cm^{1}. It can be readily assigned to the observed middlestrong band at 668 cm^{1}. The calculated Cartesian atomic displacements of this mode suggest it to be v_{27}, the inpane deformation of the naphthyl ring mainly involving the bending of C7C8C9/C10C5C6 bond angles.
In the lowfrequency region, two strong Raman band was observed at 577 and 528 cm^{1}, respectively. We assign 528 cm^{1} band to v_{28}, the inplane naphthyl deformation mainly involving the translational separation of two benzo rings of naphthyl, which is calculated at 530 cm^{1}. In the calculated Raman spectrum, a moderately strong band was predicted at 577 cm^{1}, which, according to the calculated atomic Cartesian displaces, can be assigned to the outofplane deformation of the naphthyl ring γ_{9} mixed mildly with inplane vibration like v_{29}. Another lowfrequency Raman band observed at 436 cm^{1} is thought corresponding to the calculated band at 433 cm^{1}, which can be assigned to the inplane deformation of the naphthyl ring v30' combined with a γ_{9}like outofplane deformation.
According to our calculation, the most of the strong bands observed in nonresonance Raman belong to A symmetry, with the exceptions of the 1358 cm^{1} band that belongs to B symmetry and the 1618 cm^{1} band that is composed of two modes belong to A and B symmetries respectively.
3. UV resonant Raman (UVRR) spectrum excited at 325 nmFigure 5(a) displays the resonance Raman spectrum (RRS) of BINAM excited with UV light at 325 nm, where the wavelength of the incident light is nearly in resonance with the electronic absorptions of BINAM at 348 nm. As shown in Fig. 5(a), evident RR bands were observed at 1618, 1595, 1567, 1475, 1438, 1380, 1358, 1289, 1277, 1213, 1158, 1147, 1024, 851, 668, 579, and 528 cm^{1}, most of which also appear in the 488 nm excited normal Raman with considerably intensities. However, in comparison with the normal Raman (Fig. 4(a)), the RR bands at 1618, 1475, 1358, 1289, 1158 cm^{1} show clear increase in relative intensities. Especially, the RR band 1618 cm^{1} displays greatly enhancement, which is even stronger than the 1380 cm^{1} band, the strongest band in normal Raman.
Theoretical simulation of UVresonance Raman spectrum (Fig. 5(b)) was carried out with frequencydependent polarizability using coupledperturbation theory. Experimentally, the resonance Raman spectrum was recorded by using the 325 nm line of a HeCd laser as the excitation source, which is blueshifted by 2034 cm^{1} (0.252 eV) from the UV absorption band (348 nm). This energy difference must be considered in calculation model since it is known that the resonance Raman intensities sensitively depend on the wavelength difference between the incident laser line and the electronic absorption maximum. Therefore, the energy of the incident light was set as 4.01 eV, blue shifted by 0.252 eV with respect to the TDDFT calculated UVabsorption peak, for theoretical simulation of UVRR. As shown in Fig. 5, the calculations well reproduce the enhancement pattern of UV nearresonance Raman spectrum. It is noticed that the calculations predicted increased relative intensities for the 1624, 1474, 1351, 1282, 1158 cm^{1} bands of the UVRR spectrum (Fig. 5(b)) as compared with theoretical normal Raman (Fig. 4(b). This is consistent with the experiment observations that the corresponding bands (1618, 1475, 1358, 1289, 1158 cm^{1}) are enhanced in 325 nm excited spectrum (Fig. 5(a)) as compared with 488 nm excited one (Fig. 4(a)). Especially, the 1624 cm^{1} band was predicted to be significantly enhanced by UV excitation. Meanwhile, the 1381 cm^{1} band remains as one of the strongest bands, which is also consistent with the experiment. According to the calculations, the 1624 cm^{1} band in the Raman spectrum is composed of two modes, i.e., 1625 cm^{1} (v_{7}+NH_{2} scissoring) and 1624 cm^{1} (v_{7}+NH_{2} scissoring), and their intensity ratio is about 1:2.1. Thus one can reasonably considered that the intensity of the 1618 cm^{1} band in Fig. 5(a) comes mainly from the B symmetric γ_{7} mode (with mixing of NH_{2} scissoring). The enhancement pattern of UV resonant Raman of BINAM may result from both the nonzero FranckCondon overlap due to excited state pseudoJahnTeller distortion and the vibronic coupling between the excited states, which are discussed in the next section.
IV. DiscussionResonance Raman is much different from normal nonresonance Raman in physical mechanisms. For resonance Raman in which the incident irradiation is resonant with an electronic transition, Raman signals can be enhanced either due to the nonzero FrankCondon overlap between the ground and the resonant excited state (Aterm mechanism) or due to the vibronic coupling between the electronic states (Bterm mechanism) [4145]. Unless the vibrational wave functions are nonorthogonal, the vibrational overlap integrals are zero and the Aterm mechanism dose not contribute Raman intensities. For any vibrational mode of a molecule, nonorthogonality of these wave functions will subsist if, between the ground state and excited states, there is either a displacement of the potential energy minimum along the normal coordinate, or a difference of vibrational frequency, i.e. a change in shape of the potential energy surface. Symmetry arguments indicate that such a displacement may only occur for totally symmetric modes unless the molecular symmetry is altered in the excited state. However, if a change of molecular symmetry accompanies the electronic transition, this restriction is relieved. In this case, Raman bands attributed to nontotally symmetric fundamentals may acquire intensity under resonance conditions from either the A or Bterms of the Raman polarizability. Nevertheless, it is generally the case that the Aterm contribution is much more important for totally symmetric fundamentals when the incident radiation is in resonance with a strong electricdipoleallowed electronic transition. The domination of the totally symmetric modes (A modes) in the UVRR of BINAM, as manifested by the strong intensities of 1380, 1475, 1024, 851, 668, and 528 cm^{1} bands in Fig. 5(a), may be considered as the consequence of Aterm enhancement.
In Fig. 5(a), several nontotally symmetric modes (B modes), such as the 1618, 1358, 1289, and 1158 cm^{1} bands, were also measured with appreciable intensities. Clark and Dines [42] have pointed out that RR bands attributed to nontotally symmetric fundamental vibrations may acquire intensity via three mechanisms: (i) Aterm activity due to a change of molecular symmetry in the resonant electronic state; (ii) Aterm activity due to excitedstate JahnTeller distortion; (iii) Bterm scattering, involving the vibronic coupling of the resonant state to a nearby electronic excited state. The mechanism (ii) can be considered as a specific case of mechanism (i), and it has been used to explain to the Qband excited resonant Raman of the porphyrin compounds, in which the nontotally symmetric modes (B_{1g} and B_{2g}) have been found dramatically enhanced due to the excited state JahnTeller distortion [46, 47]. We consider that both the mechanisms (i) and (iii) are responsible for the enhancement of the nontotally symmetric modes in the UVRR of BINAM for the following reasons.
Firstly, as mentioned above, the broad 348 nm band in the UV absorption spectrum of BINAM can be attributed to the overlap of three electronic transitions, i.e., 1^{1}B, 2^{1}B, and 2^{1}A. These transitions correspond to the electronic excitation from HOMO, HOMO1 to LUMO, LUMO+1. The orbital energies of HOMO2, HOMO1, HOMO, LUMO, LUMO+1, LUMO+2 are 6.354, 5.540, 5.492, 1.253, 1.179, 0.428 eV, respectively, indicating that the four frontier molecular orbitals, i.e., HOMO, HOMO1 to LUMO, LUMO+1, are well separated from other orbitals. Moreover, the HOMO/HOMO1 and LUMO/LUMO+1 are nearly degenerate, which hints a pseudoJahnTeller distortion may occur for the electronic configurations (37b)^{1}(39a)^{1} and (38a)^{1}(38b)^{1}. Under this circumstance, the two configurations, (37b)^{1}(39a)^{1} and (38a)^{1}(38b)^{1}, can be effectively coupled by certain nontotally symmetric modes. If a molecule undergoes a change of symmetry upon excitation then the Aterm active modes are those that are totally symmetric in the subgroup formed by the symmetry operations common to the ground and excitedstate point groups (common group). The B modes of BINAM, while they are nontotally symmetric under the ground state point group (C_{2} group), are totally symmetric under the distorted C_{1} group at the excited states. Accordingly, these modes can be enhanced via the Aterm mechanism.
Secondly, nontotally symmetric modes (B modes of BINAM) can be enhanced via the Bterm mechanism. According to Ref.[42], the Bterm of Raman polarizability is proportional to the vibronic coupling integral between two excited states (
Furthermore, the dramatic enhancement for the 1618 cm^{1} (experimental) band may be intuitively understood by the general empirical rule that the vibrational coordinates responsible for converting a molecule from its ground state equilibrium conformation to the excitedstate geometry will give rise to resonanceenhanced Raman bands (Tsuboi's rule) [48]. For BINAM, the lone pair of NH_{2} group can interact with the naphthyl πsystem through hyperconjugation. The electronic excitations from the ground to the 1^{1}B and 2^{1}B states induce an obvious redistribution of electron density for the NH_{2} groups and the naphthyl rings, which can result in large changes for the CN bond distances and the NH_{2} bongangles. Our DFT calculation manifests that the 1624 cm^{1} (calculated) mode contains a significant contribution from the NH_{2} bending and a moderate one from CN stretching, thus according to Tsuboi's rule, 1624 cm^{1} band is resonantly enhanced with the UV excitation.
V. ConclusionWe have studied the IR absorption, visible excited normal Raman, and UVexcited nearresonance Raman spectra of 1, 1'binaphthyl2, 2'diamine (BINAM). Density functional theory (DFT) calculations were carried out to study the vibrational frequencies and the groundstate structure of BINAM. The measured IR and Raman spectra of BINAM were found in good accordance with the calculations. The assignments of observed IR and Raman bands were proposed on the basis of the calculated and measured frequencies and intensities. In comparison with the visible excited normal Raman, several bands of BINAM were found dramatically enhanced in the UV resonance Raman spectrum. Possible excited state structure distortion was discussed based on the RR intensity analyses.
Supplementary materials: Atomic cartesian displacements for the normal modes of 1chloro2naphthylamine are shown.
VI. AcknowledgmentsThis work was supported by the National Natural Science Foundation of China (No.21273211, No.21573208), USTCNSRL Association Foundation (No.NSRLLHJJ(1415012), and the Supercomputation Center of USTC.
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b. 中国科学技术大学国家同步辐射实验室, 合肥 230026