Chirality transition in the epoxidation of (�)-a-pinene and successive

hydrolysis studied by Raman optical activity and DFTw

Shi Qiu,zab Guanna Li,zab Peng Liu,ab Changhao Wang,ab Zhaochi Feng*a and

Can Li*a

Received 25th September 2009, Accepted 12th January 2010

First published as an Advance Article on the web 9th February 2010

DOI: 10.1039/b919993d

Characterization of the chirality evolution involved in chemical and biochemical reaction

processes is extremely important to the understanding of the chiral catalysis mechanism. In this

work, the chiral transition from the epoxidation of (�)-a-pinene to a-pinene oxide and successive

hydrolysis to (�)-pinanediol has been studied as an archetype of the asymmetric catalysis by

Raman optical activity (ROA) and the DFT calculation. Minor changes of the absolute

configuration of the chiral products from (�)-a-pinene to (�)-pinanediol lead to the dramatic

variation in ROA spectra indicating that the chirality is delocalized in the whole molecule rather

than only concentrated on the chiral centers. The oxygen atom of a-pinene oxide contributes

strong ROA signals while the two hydroxyl groups of (�)-pinanediol give no apparent

contribution to the chirality in terms of ROA signals. Isolation of the two symmetric anisotropic

invariants shows that the predominant contribution to the ROA signals stems from the electric

dipole–magnetic dipole invariant, and the bond polarizability model is indeed found to be a good

approximation for molecules composed of entirely axially-symmetric bonds in a-pinene oxide and

(�)-pinanediol. This study demonstrates the feasibility of using ROA to sensitively monitor the

variation of the chirality transition during the chiral reactions either in the chemical or biological

system.

Introduction

Direct observation of configurational and conformational

transitions is crucial for understanding the mechanism of

chiral reactions, especially in chemical and biochemical

catalysis. However, these changes are difficult to probe using

conventional methods, although dynamics simulations

sometimes help.1 Chiroptical spectroscopic characterization,

especially vibrational optical activity, including vibrational

circular dichroism (VCD) and Raman optical activity

(ROA), are proven to be an effect way to determine the

configuration, conformation and behavior of chiral

molecules.2–6 In particular, ROA is considered as an ideal

tool for the study of biological molecules in aqueous

solution.7–14

The theoretical background for the ROA phenomenon was

given by Barron and Buckingham in 1971.15 Polavarapu

presented the first complete theoretical study of the CIDs

observed in experimental ROA spectra in 1990,16 using a

static approximation to G0(o) as proposed by Amos.17 The

first correlated gauge-origin independent calculations were

presented by Helgaker et al. using multi-configurational

self-consistent field (MCSCF) wave functions and London

atomic orbitals.18 Bour19 has combined sum-over-states

DFT calculations (including hybrid functionals) of optical

tensors with numerical differentiation with respect to nuclear

displacements. The analytical DFT calculations of ROA

spectra using linear response theory and London atomic

orbitals was presented by Ruud et al.20 A quantum-mechanical

approach for predicting the ROA spectra is essential for

an in-depth interpretation of the experimental spectra,

providing helpful physical insight into the generation of

vibrational optical activity,21–26 and it has been successfully

applied to the studies on small chiral molecules such

as bromochlorofluoromethane,27,28 substituted oxiranes,

biologically significant amino acids and oligopeptides as

primary model molecules for understanding the stereochemical

properties of polypeptides and proteins.29–34

Oxyfunctionalization of cyclic olefins is of industrial and

biological importance due to the possibility of transforming

cheap and readily available substrates to valuable inter-

mediates for fine chemicals and pharmaceutical synthesis.35–43

a-Pinene is a standard molecule frequently used in ROA study.

We chose epoxidation of a-pinene to a-pinene oxide and

a State Key Laboratory of Catalysis, Dalian Institute of ChemicalPhysics, Chinese Academy of Sciences, Dalian, 116023, China.E-mail: zcfeng@dicp.ac.cn, canli@dicp.ac.cn;Fax: +86-411-84694447; Tel: +86-411-84379303,+86-411-84379070

bGraduate University of Chinese Academy of Sciences, Beijing,100049, China

w Electronic supplementary information (ESI) available: Synthesisprocedure of a-pinene oxide; introduction of visualization of vibra-tional modes and atomic contribution patterns (ACPs) pictorial; XRDand IR spectra of catalyst ZnAl-[Salen(Mn)]; NMR spectrum ofa-pinene oxide; vibrational modes and ACPs pictorial representation;Mulliken atomic charges distribution; Raman vibrational analysis andanisotropic invariants of the optical activity tensor components. SeeDOI: 10.1039/b919993dz These authors contributed equally to this work.

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subsequent hydrolysis as an archetypal reaction. These reactions

are proper models for assessing the chiral structure changes in

the chemical and biochemical reactions.

Spectroscopic characterization of the absolute configuration

changes is a long standing challenge in chiral catalysis. To our

knowledge, there is no report on the chiral characterization in

the organic synthesis or asymmetric catalysis processes using

ROA spectroscopy. In this work, we studied the chirality

transition in the epoxidation of (�)-a-pinene to a-pinene oxideand the successive hydrolysis to (�)-pinanediol as a typical

reaction in the asymmetric catalysis process. The ROA

differences between the three bicyclo compounds, owing to

the addition of the two new chiral centers and the contribution

from the substituent oxygen atom and hydroxyl groups, are

discussed in detail. Based on the DFT calculations, it is found

that the main contributions to the backscattering ROA

intensities are from the electric dipole–magnetic dipole

symmetric anisotropic invariant while those from the electric

dipole–electric quadrupole invariant are relatively small. This

work also demonstrates that ROA can supply information on

the chiral environment changes and transitions during the

chiral catalytic reactions either in chemical or biological

systems.

Method

Experimental

The chiral transition model is represented by the epoxidation

of (�)-a-pinene and successive hydrolysis to (�)-pinanediol.(�)-a-Pinene and (�)-pinanediol were purchased from

Acros and used without further purification. a-Pinene oxide

was synthesized by the method of Anderson.41 Catalyst

characterization was shown in Fig. S1.w a-Pinene oxide

structure was confirmed by 1H NMR spectrum (in CDCl3)

shown in Fig. S2,w with 83.0% de confirmed by GC-MS.

ROA spectra were recorded on a Chiral RAMAN instru-

ment (BioTools Inc.), which simultaneously provided both

backscattering Raman and scattered circularly polarized ROA

spectra. The instrument utilized a CW 532 nm laser source

(Verdi 2) and a back-thinned CCD detector, which was

optimized for recording spectra in the range 100–2500 cm�1.

Spectral resolution was approximately 7 cm�1. The power at

the laser head output was 600 mW for both (�)-a-pinene anda-pinene oxide and 350 mW for (�)-pinanediol in CCl4solution (4 M). The total exposure time was 18 min for

both (�)-a-pinene and a-pinene oxide, and 137 min for

(�)-pinanediol.As an appropriate experimental quantity, the dimensionless

circular intensity difference (CID) first introduced by Barron15

is defined as

D ¼ IR � IL

IR þ IL

where IR and IL are the scattered intensities in right- and

left-circularly polarized incident light, respectively. Using

the Placzek polarizability approximation, differential CIDs

associated with the four most important scattering geometries

for an isotropic collection of chiral molecules for scattered

circular polarization forms of ROA can be written as:12

Dð0�Þ ¼ 8½45aG0 þ bðG0Þ2 � bðAÞ2�2c½45a2 þ 7bðaÞ2�

Dð180�Þ ¼ 48½bðG0Þ2 þ ð1=3ÞbðAÞ2�2c½45a2 þ 7bðaÞ2�

Dxð90�Þ ¼12½45aG0 þ 7bðG0Þ2 þ bðAÞ2�

c½45a2 þ 7bðaÞ2�

Dzð90�Þ ¼12½bðG0Þ � ð1=3ÞbðAÞ2�

6cbðaÞ2

a and G0 are the isotropic invariants of the polarizability

tensor and the electric dipole–magnetic dipole optical activity

tensor, respectively.

a =1

3aaa =

1

3(axx + ayy + azz),

G0 ¼ 1

3G0aa ¼

1

3ðG0xx þ G0yy þ G0zzÞ;

b(a)2, b(G0)2 and b(A)2 are the anisotropic invariants of the

polarizability tensor and optical activity tensor components.

b(a)2 =1

2(3aabaab � aaaabb),

bðG0Þ2 ¼ 1

2ð3aabG0ab � aaaG0bbÞ;

b(A)2 =1

2oaabeagdAgdb,

Calculations

For the DFT calculation, the program package Gaussian 03

version D 0144 was used. The geometry optimization of

isolated (�)-a-pinene, a-pinene oxide and (�)-pinanediol andthe simulation of the Raman and ROA spectra were

operated using the Becke-three-parameter-Lee–Yang–Parr

(B3LYP)45–47 hybrid functional and the aug-cc-PVDZ basis

set.48–51 We evaluated the relative energy of the two possible

configurations of a-pinene oxide to confirm the abundant one

(upper part of Fig. 1). The result shows that configuration I is

more stable than II with an energy difference of 15.75 kJ mol�1,

so the configuration of the epoxide product should be mainly

in configuration I which is consistent with the de% value.

Comparison of the B3LYP/aug-cc-PVDZ ROA spectra of the

two isomers (lower part of Fig. 1) to the experimental ROA

spectrum of a-pinene oxide (middle part of Fig. 3) leads

unambiguously to the conclusion that the trans-isomer is

mainly the configuration of a-pinene oxide deriving from

(�)-a-pinene. For (�)-pinanediol, the two hydroxyl groups

hold cis form attached to C(2) and C(3). The structures and

atom numbering for (�)-a-pinene, trans-a-pinene oxide and

(�)-pinanediol are shown in Fig. 2. The optimized geometry

parameters are listed in Table 1, and the Mulliken atomic

charges distribution and the vibrational modes analysis in

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Tables S1, S2, S3 and S4 in the ESI,w respectively. The

symmetric anisotropic electric dipole–magnetic dipole invariants

b(G0)2 and electric dipole–electric quadrupole invariants b(A)2

are given in Table S5.wThe pyVib2 program was for the analysis of the vibrational

modes of Raman as well as the atomic contribution patterns

(ACPs) pictorial, wherein quantities attributable to individual

atoms are drawn directly onto the molecular structure from

Gaussian outputs.52,53 In the vibrational mode, the direction

of atomic displacements is perpendicular to the junction plane

between the two hemispheres, shaded yellow or blue, while

their amplitudes are proportional to the radius of the sphere;

in ACPs, red for positive and yellow for negative contribution

to the ROA signals, and the surface area of each sphere is

proportional to ROA scattering section.

Results and discussion

Stereostructure changes during chirally catalytic reactions

Fig. 3 (upper part) shows the typical chiral catalytic route of

epoxidation of (�)-a-pinene to a-pinene oxide41 and then

hydrolysis to (�)-pinanediol. Their corresponding Raman

and ROA spectra are presented in the lower part of Fig. 3.

For the epoxidation and the subsequent hydrolysis, the chiral

configurations of C(1) and C(5) remain unchanged, while the

achiral CQC transforms to the C–O–C epoxide and finally

the diol, producing two new chiral centers at C(2) and C(3).

The apparent distinction is located in the B1660 cm�1 region

of the CQC stretching mode in the Raman spectra. Fig. 4

shows the computational simulation results. The calculated

Raman and ROA results are in good agreement with the

experimental data. Due to the additional stereo-specific

information contained in ROA, in some cases it resolves

overlaps in the parent Raman spectra into the bands of

different signs or different relative intensities, and so helps to

assign the vibrational modes more clearly. From Table 1, the

structural differences among the three monoterpene-based

compounds are mainly from the skeletal six-membered ring,

especially in the vicinity of the C(2)–C(3) bond. The bond

lengths of C(2)–C(3), C(2)–C(10) and C(3)–C(4) gradually

increase from (�)-a-pinene to (�)-pinanediol, and the position

of branched C10 relative to the six-membered ring changes

markedly. The variation of absolute configuration combined

with the Mulliken atomic charge distribution (Table S1w)implies the change of the polarizability and optical activity

tensor components and consequently the ROA profile. Besides

the CQC stretching and twisting modes of (�)-a-pinene at

1660 (mode 56) and 571 (mode 14) cm�1 and the characteristic

oxygen atom involved, the modes of a-pinene oxide at 865

(mode 22), 845 (mode 21), 820 (mode 20), 772 (mode 19) and

700 (mode 18) cm�1, the (�)-(+)-(�) triplet of (�)-a-pinene inthe region of 1480–1400 cm�1 turn to the (�)-(+) couplet of

a-pinene oxide and (�)-pinanediol. To discuss Raman and

ROA spectra in detail, the vibrational modes are divided into

four ranges.54,55

ROA of C(10)H3 antisymmetric deformation induced

by the additional chiral centers

In the region of 1500–1400 cm�1, assigned to CH2 scissoring

and antisymmetric CH3 deformation, a similar negative–

positive couplet (negative at high wavenumbers and positive

at low wavenumbers) centered at ca. 1470 cm�1 appears in the

three ROA spectra. While in the Raman spectra, only an

intense broad band with a shoulder band is observed for both

(�)-a-pinene and (�)-pinanediol, and two split bands for

a-pinene oxide. There is another strong negative band at

1438 cm�1 (mode 50) in the ROA spectrum of (�)-a-pinene,while it is absent in the spectra of a-pinene oxide and

(�)-pinanediol.In Fig. 5, the similar negative–positive couplet centered

at ca. 1470 cm�1 comes from different atom vibrational

distributions. The negative band of (�)-a-pinene at

1477 cm�1 (modes 55 and 53) comes from C(8)H3 and

C(9)H3 antisymmetric deformation and C(6)H2 and C(4)H2

scissoring, the negative band of pinene oxide at 1472 cm�1

(modes 59 and 57) is thought to be C(6)H2 scissoring, C(10)H3

antisymmetric deformation and C(2)–C(3) stretching, and the

negative band of (�)-pinanediol at 1476 cm�1 (mode 64) is

mainly assigned to C(8)H3, C(9)H3 and C(10)H3 anti-

symmetric deformation. The positive band of (�)-a-pineneat 1452 cm�1 (mode 52) is ascribed to C(4)H2 scissoring and

Fig. 1 Two configurations of a-pinene oxide (upper) and their

simulated Raman and ROA spectra (bottom). The Raman and

ROA spectra were plotted with a Lorentzian band shape with a full

width at half-maximum height of 7 cm�1.

Fig. 2 Atom numbering of (�)-a-pinene, a-pinene oxide and

(�)-pinanediol (lower, red for oxygen, gray for carbon and white for

hydrogen atom).

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C(9)H3 antisymmetric deformation, the positive band of

pinene oxide at 1440 cm�1 (modes 55 and 54) is thought to

be C(4)H2 scissoring, C(8)H3, C(9)H3 and C(10)H3 anti-

symmetric deformation and C(2)–C(3) stretching, and the

positive band of (�)-pinanediol at 1461cm�1 (mode 62) is

ascribed to C(9)H3 and C(10)H3 antisymmetric deformation

and C(4)H2 scissoring modes. The unique sharp negative band

of (�)-a-pinene at 1438 cm�1 (mode 50) is mainly ascribed to

C(4)H2 scissoring and C(8)H3 and C(9)H3 antisymmetric

deformation. Although no apparent similar negative bands

are observed for pinene oxide and (�)-pinanediol, there

exists a negative band in the simulated ROA spectrum of

Table 1 Calculated skeletal bong lengths (A), bond angles (deg) and dihedral angles (deg) for the three molecules (# for distinct variation)

Entry (�)-a-Pinene a-Pinene oxide 2,3-(�)-Pinanediol Note

C1–C2 1.522 1.534 1.539C2–C3 1.343 1.476 1.582 #C2–C10 1.502 1.508 1.531 #C3–C4 1.515 1.526 1.559 #C4–C5 1.541 1.541 1.536C5–C6 1.556 1.555 1.550C5–C7 1.574 1.576 1.569C6–C1 1.562 1.553 1.556C7–C1 1.582 1.582 1.579C7–C8 1.529 1.532 1.533C7–C9 1.536 1.535 1.538C1–C2–C3 116.6 114.0 110.9 #C2–C3–C4 119.9 117.9 114.4 #C3–C4–C5 110.3 111.5 113.6 #C4–C5–C6 108.6 109.1 107.8C5–C6–C1 85.9 86.4 86.4C6–C1–C7 87.1 87.5 87.3C6–C1–C2 107.0 109.7 108.2C1–C7–C5 84.6 84.7 85.0C7–C5–C6 87.6 87.6 87.9C10–C2–C3–C4 �178.5 �150.9 �140.8 #C10–C2–C1–C6 �133.6 �161.5 �178.3 #

Fig. 3 The typical chiral catalytic route of epoxidation of (�)-a-pinene to a-pinene oxide41 and then hydrolysis to (�)-pinanediol (upper) and the

backscattered SCP Raman and ROA spectra of (�)-a-pinene (left part), a-pinene oxide (middle part), and (�)-pinanediol solved in CCl4 (right

part). denotes the chiral centers C(1) and C(5), and denotes the new chiral centers, C(2) and C(3). The numbers label the vibrational modes.

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pinene oxide (mode 52), and it is assigned to C(4)H2

scissoring, C(8)H3, C(9)H3, and C(10)H3 antisymmetric

deformation modes.

In this region, C(4)H2 scissoring and C(8)H3, C(9)H3

antisymmetrical deformations dominate the ROA bands of

(�)-a-pinene, and after epoxidation of the CQC bond, the

Fig. 4 The computational simulation of Raman and ROA spectra of (�)-a-pinene (left part), trans-a-pinene oxide (middle part), and

(�)-pinanediol (right part). The Raman and ROA spectra were plotted with a Lorentzian band shape with a full width at half-maximum height

of 7 cm�1.

Fig. 5 Vibrational normal modes (left) and ACPs (right) for (�)-a-pinene (left column), a-pinene oxide (middle column) and (�)-pinanediol(right column) assigned to CH2 scissoring and antisymmetric CH3 deformation (vibrational mode number and the ROA signs are labeled).

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C(10)H3 antisymmetrical deformation shows Raman optical

activity. As the C(10) is adjacent to one of the newly formed

chiral center C(2), it implies that the additional chiral

environment induces the ROA of C(10)H3 antisymmetrical

deformation. It can be also seen from modes 51 and 48 of the

calculated ROA spectrum of (�)-a-pinene, the ACPs

(Fig. S3w) shows that although the two hydrogen atoms of

the C(10)H3 group both have strong ROA contributions, their

signs are opposite, so the total contribution is nearly zero and

the antisymmetrical deformation of C(10)H3 group shows

little ROA in (�)-a-pinene. Overall, the similar negative–

positive couplet in the ROA spectra reflects the chiral

environment of the bicyclic structure.

ROA distributions among similar vibrational modes

In the 1400–1300 cm�1 region, Raman bands are mainly from

the vibrations of CH2 wagging and symmetric CH3 deformation.

In Fig. 6, the positive band of (�)-a-pinene at 1379 cm�1

(mode 46) in the ROA spectra is ascribed to C(10)H3

symmetric deformation. Its counterpart for a-pinene oxide is

at 1396 cm�1 (mode 49) and is nearly absent for (�)-pinanediol(mode 56). The relative position of C(10)H3 in the chiral

environment gradually transforms, and the ROA changes

although they have similar Raman vibrational modes.

A similar situation is also found for the positive band at

1270 cm�1 of (�)-a-pinene (mode 41), the band at 1283 cm�1

of a-pinene oxide (mode 44) and the band at 1224 cm�1 of

(�)-pinanediol (modes 49) involving the C(1)H and C(5)H

wagging, C(4)H2 twisting and C(7)–C(5) stretching modes

(Fig. 7). The relatively large shift in the band of (�)-pinanediolis thought to be the involvement of the OH wagging and

C(6)H wagging vibrations. Although they all show one

positive band, their ROA signals from the atom contributions

are not the same. The predominant contribution to the

positive ROA signal is from C(7) for (�)-a-pinene and the

two hydrogen atoms linked to C(4) make almost exactly

opposite contribution. For a-pinene oxide, the hydrogen atom

linked to C(3) and one of the hydrogen atoms linked to C(4)

make comparable contributions to ROA with that of C(7).

While for (�)-pinanediol, C(5), C(2), the hydrogen atom

linked to C(3) and one of the hydrogen atoms linked to C(6)

make relatively large positive contributions and the hydrogen

atom linked to C(5) make strong negative contribution

although the contribution of C(7) is still positive.

Apparently, the similarity of the vibrational modes comes

from their similar bicyclic structure, however, the optical

activity tensor derivatives with respect to the normal coordinate

vary in the slightly different asymmetric environments. Thus

the atomic contributions to ROA differ obviously and produce

ROA signals with the same or opposite signs. Some other

vibrational modes alike are shown in Fig. S4a–d.w

ROA induced by substituted oxygen atom and diol

In the region 1200–600 cm�1, all the three monoterpene-based

compounds show very characteristic strong ROA. For the

a-pinene oxide, the substituent oxygen atom makes a large

contribution to the ROA signals in this region. In Fig. 8, it is

easy to observe positive or negative effect from the oxygen

atom, which is in good agreement with the sign of related

ROA bands for a-pinene oxide. For example, the strongly

negative band at 865 cm�1 (mode 22) is ascribed to

C(2)–O–C(3) ring deformation. The ACPs suggest that the

substituted oxygen atom contributes negatively to the ROA

intensity. Similarly, in the next ring deformation and H

bending mode at 845 cm�1 (mode 21), the oxygen atom has

a positive contribution to the whole ROA intensity which is

consistent with the observed strong positive band. The negative

contribution from the oxygen atom in the skeletal deformation

and branched CH3 wagging mode at 820 cm�1 (mode 20) is

also in accordance with the strong negative band of the

experimental spectrum. The same correlation is observed for

the positive band at 772 cm�1 (mode 19) assigned to the ring

deformation, and other modes at 1144 (mode 36), 1066 (mode 32),

1045 (mode 31), 700 (mode 18) and 572 cm�1 (mode 15)

as well. The excellent correlation of the contributions of

substituted oxygen atom with the sign of ROA signals is

distinct to a-pinene oxide.

Fig. 6 Vibrational normal mode (left), ACPs (middle), b(G0)2 and

b(A)2/3 invariants (right) for (�)-a-pinene mode 46 (upper), a-pineneoxide mode 49 (middle), and (�)-pinanediol mode 56 (lower)

(the position of atom C(10) is labeled).

Fig. 7 Vibrational normal mode (left), ACPs (middle), b(G0)2 and

b(A)2/3 invariants (right) for (�)-a-pinene mode 41 (upper), a-pineneoxide mode 44 (middle), and (�)-pinanediol mode 49 (lower) (the

position of atoms C(2), C(3), C(4), C(5), C(6) and C(7) is labeled).

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In contrast, the contributions from the two hydroxyl groups

of the diol are relatively small and have opposite signs in some

bands (Fig. S5w). For example, the contributions from the two

oxygen atoms of the diol to the positive band at 1136 cm�1

(mode 41) are relatively small and opposite in amplitude.

Similarly, the contributions from the two hydrogen atoms of

the diol to the positive band at 1052 cm�1 (mode 37) are also

relatively small and opposite, although the amplitude is not

the same. Thus, the correlation between the hydroxyl group

contributions and ROA signals is not as direct as that of the

oxygen atom in the a-pinene oxide. This can be explained in

terms of the rigid structure of the newly formed three-member

ring C(2)–O–C(3) in the asymmetric environment of a-pineneoxide, whereas the small ROA activity of hydroxyl group in

the chiral environment of (�)-pinanediol in the region of

1400–600 cm�1 and the cancelling effect from the two hydroxyl

groups adjacent to chiral carbon atoms of opposite configuration

(C(2) takes R configuration and C(3) takes S configuratrion).

Although the ROA signals below 600 cm�1 of a-pineneoxide and (�)-pinanediol are relatively weak, there is still a

significant band for (�)-a-pinene, i.e. a negative band at

566 cm�1 (mode 14) corresponding to the CQC twisting which

is absent for a-pinene oxide and (�)-pinanediol.

Delocalization of chirality represented by ROA

Fig. 7 shows that the atomic contributions to the ROA signals

are different while the vibrational modes are similar (as

discussed above and also in Fig. S4a–dw). It may be inferred

that ROA signals are very sensitive to minor changes of the

chiral environment, even if the vibrational modes seem alike,

as the bicyclic structure has little change. The two new formed

chiral centers from the epoxidation of (�)-a-pinene change theoptical activity not only of the adjacent groups, but also of the

whole chiral molecules, especially the bicyclic structure.

Furthermore, in Fig. 9, the groups not linked directly to the

chiral centers, i.e. C(8)H3, C(9)H3 and C(10)H3 in (�)-a-pinene, and C(8)H3 and C(9)H3 both in a-pinene oxide and

(�)-pinanediol, show medium or strong ROA activity in some

modes, especially in the region below 1200 cm�1 involving the

mixed motions of branched and skeletal parts. The C(8)H3,

C(9)H3 and C(10)H3 groups are away from the local chiral

centers C(1) and C(5) in (�)-a-pinene, and after epoxidation,

C(10)H3 is adjacent to the new-born chiral center C(2), yet the

C(8)H3 and C(9)H3 groups are still distant from the four chiral

centers C(1), C(2), C(3) and C(5) in a-pinene oxide and

(�)-pinanediol. Similar situations are also observed for the

modes 55 of (�)-a-pinene and 62 in (�)-pinanediol in Fig. 5.

These phenomena are most likely attributable to the

delocalization of chirality in the whole molecule rather than

just concentrated on the chiral centers themselves.

Contribution of electric dipole–magnetic dipole to ROA

The differential Raman backscattering intensity for the SCP

form of ROA contains the symmetric anisotropic electric

dipole–magnetic dipole invariants b(G0)2 and electric dipole–

electric quadrupole invariants b(A)2. Both the seperated

optical acitivity tensor invariants of the three bicyclic compounds

are evaluated in Fig. 10 (and the isolated invariants in Fig. S6

and Table S5w) and the relative ratio of b(G0)2 and b(A)2 in

some modes are shown in Fig. 6 and 7. In the region

1800–500 cm�1, the contributions of the b(A)2 are generally

much smaller than those of the b(G0)2, so the symmetric

anisotropic electric dipole–magnetic dipole invariant b(G0)2

plays a predominant role in the generation of ROA signals.

This phenomenon is also found in other small chiral molecules,

e.g. (S)-methyloxirane, (S)-glycidol and (+)-trans-pinane.56,57

The exceptions are only for few cases, for example in Fig. S6,wthe mode 42 in (�)-a-pinene, the mode 45 in a-pinene oxide, themodes 61, 59, 54, 51 and 48 in (�)-pinanediol mainly ascribed

to C–H wagging. However, their ROA signals are very weak.

For both of a-pinene oxide and (�)-pinanediol, b(G0)2 and

b(A)2 are nearly equal in many modes, yet not the case in

a-pinene, for which even the signs of b(G0)2 and b(A)2 are

opposite. In the bond polarizability model of ROA,13 if all the

bonds are axially-symmetric, the b(A)2 contribution will be

exactly 1/3 that of the b(G0)2. Since the bonds are all axially

symmetric in the a-pinene oxide and (�)-pinanediol (but

perhaps with some distortion in the oxide), but not in

a-pinene, this approximation should hold much better for

the oxide and diol. Such studies show that it is indeed found

to be a good approximation for molecules composed of

entirely axially-symmetric bonds, then the speed of ab initio

ROA calculations could be significantly increased, thereby

facilitating the application to some quite large systems, e.g. all

the bonds in oligo and polysaccharides are axially symmetric.

Conclusions

For the first time, the chirality transition in the epoxidation of

(�)-a-pinene to a-pinene oxide and subsequent hydrolysis to

(�)-pinanediol as an archetype of the asymmetric catalysis has

been studied by ROA and DFT calculations.

After epoxidation, the rigid bicyclic structure of (�)-a-pinene persists and two new chiral centers are born. The new

chiral centers induce the ROA signals of adjacent C(10)H3

antisymmetric deformation. The similar ROA couplet in the

region of 1480–1400 cm�1 associated with CH2 scissoring and

Fig. 8 The ACPs pictorial representation of ROA intensities of

a-pinene oxide (the position of atom O(27) is labeled).

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antisymmetric CH3 deformation may reflect the chiral

environment of the bicyclic structure.

The epoxidation and subsequent hydrolysis do not change

the rigid skeletal structure, however, the ROA spectra are

altered dramatically. This is caused not only by an increase in

the number of local chiral centers, but also by a chirality

delocalization effect on the whole molecule, especially in the

region below 1200 cm�1 involving the mixed motions of

branched and skeletal parts.

In the characteristic region 1200–600 cm�1 associated with

ring breathing and skeletal motions, the substituted oxygen

atom in the a-pinene oxide has significant impact on the ROA

signals, and its contributions correlate with the ROA sign.

In contrast, the two hydroxyl groups in the (�)-pinanediolgive no apparent contributions. This can be explained in terms

of the small ROA activity of hydroxyl groups in the region of

1400–600 cm�1 and the cancelling effect from the two hydroxyl

groups adjacent to chiral carbon atoms of opposite configuration.

Isolation of the two symmetric anisotropic invariants shows

the predominant contributions of the electric dipole–magnetic

dipole invariant b(G0)2 to the ROA signals. It is also found

that the bond polarizability model is a good approximation

for molecules composed of entirely axially-symmetric bonds

in a-pinene oxide and (�)-pinanediol. This study also

demonstrated that the ROA spectroscopy can sensitively

monitor the variation of the chiral structures and opens up

the possibility to study chirality transitions during asymmetric

catalytic reactions.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China (NSF, Grant Nos. 20621063, 20773123).

Notes and references

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