This paper is published as part of a CrystEngComm themed issue entitled:

Dynamic behaviour and reactivity in crystalline solids

Guest Editors: Tomislav Friščić and Graeme Day University of Cambridge, UK

Published in issue 13, 2011 of CrystEngComm

Image reproduced with permission of K. Užarević Read an editorial article by Sir John Meurig Thomas, University of Cambridge: Crystal engineering: origins, early adventures and some current trends John Meurig Thomas CrystEngComm, 2011, DOI: 10.1039/C1CE90016A Other articles published in this issue include: Drug-drug co-crystals: Temperature-dependent proton mobility in the molecular complex of isoniazid with 4-aminosalicylic acid Pawel Grobelny, Arijit Mukherjee and Gautam R. Desiraju CrystEngComm, 2011, DOI: 10.1039/C0CE00842G Are glycine cyclic dimers stable in aqueous solution? Said Hamad and C. Richard A. Catlow CrystEngComm, 2011, DOI: 10.1039/C0CE00877J Solid-state synthesis of mixed trihalides via reversible absorption of dihalogens by non porous onium salts L. Meazza, J. Martí-Rujas, G. Terraneo, C. Castiglioni, A. Milani, T. Pilati, Pierangelo Metrangolo and Giuseppe Resnati CrystEngComm, 2011, DOI: 10.1039/C1CE05050H Visit the CrystEngComm website for more cutting-edge crystal engineering research

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Photo-induced structural changes in two crystal forms with different numbersof independent molecules†‡

Julia Bakowicz, Jacek Skar _zewski and Ilona Turowska-Tyrk*

Received 29th October 2010, Accepted 31st March 2011

DOI: 10.1039/c0ce00795a

Structural changes brought about by the Norrish-Yang reaction proceeding in two crystal forms of

methyl 2-{[4-(2,4,6-triisopropylbenzoyl)benzoyl]amino}-3-phenylpropanoate were monitored by

X-ray structure analysis. The S-enantiomer and the racemic mixture of the compound crystallize in the

space groups P212121 with one molecule in the asymmetric unit and Pna21 with two symmetrically

independent molecules, respectively. The changes in the cell volume for both crystal forms are of

different kinds. They were attributed not only to the photochemical reaction but additionally to a loss

of translational order in the racemic crystal. The structures determined for the partly reacted racemic

crystal revealed that two symmetrically independent molecules react at different rates. This fact was

explained quantitatively by means of differences in the free space and in the geometry of both

molecules. It was also shown that only a single o-isopropyl group took part in the reaction.

Introduction

Monitoring single-crystal-to-single-crystal photochemical reac-

tions is one of the main subjects of our present crystallographic

studies. Among intermolecular processes, the [2 + 2] and [4 + 4]

cycloadditions were in the field of our crystallographic interest.2–4

Among intramolecular processes in crystals, the Norrish-Yang

reaction is the most intensively studied by us.5–9 This paper refers

to the results for the latter process. The mechanism of the Nor-

rish-Yang reaction is presented in Scheme 1a. In the first step,

a g-hydrogen atom is abstracted and transferred to an oxygen

atom of a carbonyl group. In the second step, named as the Yang

photocyclization, the resulted 1,4-hydroxybiradical forms

a cyclobutane ring. Methyl 2-{[4-(2,4,6-triisopropylbenzoyl)-

benzoyl]amino}-3-phenylpropanoate, 1, (Scheme 1b) is one of

compounds undergoing such a reaction. In this paper we present

the structural changes proceeding in two forms of crystals of this

compound, i.e. containing only the S-enantiomer, 1a, and con-

taining the racemic mixture, 1b. The description of structural

changes for two crystal forms of a single compound has not been

known in literature.

The structures of the pure reactant and the pure product for

the S-enantiomer of 1 were already published.10 However,

Faculty of Chemistry, Wrocław University of Technology, Wybrze _zeWyspia�nskiego 27, 50-370 Wrocław, Poland. E-mail: ilona.turowska-tyrk@pwr.wroc.pl

† Part 14 of the series ‘‘Monitoring structural transformations incrystals’’. For part 13 see ref. 1.

‡ Electronic supplementary information (ESI) available. CCDCreference numbers 799253–799268. For ESI and crystallographic datain CIF or other electronic format see DOI: 10.1039/c0ce00795a

4332 | CrystEngComm, 2011, 13, 4332–4338

structures for partly reacted crystals, i.e. containing both

reactant and product molecules, have not been presented.

Structures for the racemic mixture of 1 have not been known:

neither for the pure reactant, the pure product nor partly

reacted crystals.

Scheme 1

This journal is ª The Royal Society of Chemistry 2011

Table 1 Transmittance of the filters

Transmittance (%)

Wavelengths/nm

BG-39 filter WG-320 filter

0 <320 and >680 <30055 350 33095 460 350100 — 365

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Experimental

Both N-p-(2,4,6-triisopropylbenzoyl)benzoic acid derivatives of

(DL)- and L-phenylalanine methyl esters were prepared according

to the procedures reported by Ito et al.11 using for N-acylation

racemic or enantiomeric ester of the amino acid. The spectral

properties (1H, 13C NMR, IR) of the obtained products were

identical with the reported ones.11 1a and 1b were recrystallized

from methanol and ethanol, respectively.

The general strategy of our crystallographic studies of

photochemical reactions was described elsewhere.5,8,9

Crystals 1a and 1b were irradiated using an Hg 100 W lamp

equipped with a water filter and a BG-39 or WG-320 glass filter,

respectively. Transmittance for both filters is given in Table 1.

For 1a two crystals were examined. The final refined values of site

occupation factors revealed the following content of the product:

0, 15.0(6) and 21.6(7)% for the first crystal and 28.6(8), 55.6(12),

66.9(11), 79.9(10), 81.2(10), 87.4(8), 87.9(8) and 100% for the

second object. The p-isopropyl group in the non-irradiated

crystal 1a was described as disordered with site occupancy

factors 0.44 and 0.56. In the case of 1b only one crystal was

irradiated. The determined structures revealed 16.6(6), 20.5(6),

25.5(7), 40.2(7) and 46.2(8)% of the product, respectively (the

mean values calculated on the grounds of the content for both

symmetrically independent molecules). The quality of the

remaining structures was not satisfactory. This was the result of

a steady decrease in crystal diffracting power. A serious loss of

translational order that produced a decrease in measured inten-

sities was caused by UV radiation and was observed only for

Table 2 Selected crystal data

21.6%P (1a) 66.9%P (1a) 87.9%P (1a)

Chemical formula C33H39NO4 C33H39NO4 C33H39NO4

Formula weight 513.65 513.65 513.65Crystal system Orthorhombic Orthorhombic OrthorhombiSpace group P212121 P212121 P212121a/�A 9.3769 (13) 9.3364 (13) 9.2515 (16)b/�A 10.8131 (18) 11.1836 (19) 11.479 (2)c/�A 29.707 (4) 28.721 (5) 28.054 (5)V/�A3 3012.1 (8) 2998.9 (8) 2979.3 (9)Z 4 4 4Dx/Mg m�3 1.133 1.138 1.145m/mm�1 0.07 0.07 0.07T/K 299(2) 299(2) 299(2)Reflections collected 14 655 14 635 14 549Reflections independent 3005 2980 2953Rint 0.047 0.067 0.048R, wR (F2 > 2s(F2)) 0.078, 0.215 0.092, 0.253 0.075, 0.196Drmax, Drmin/e �A

�3 0.19, �0.25 0.21, �0.15 0.19, �0.19

This journal is ª The Royal Society of Chemistry 2011

crystal 1b (owing to the different glass filter used during the

experiment). It should be added that the crystal did not change its

external size, shape, transparency and colour on UV irradiation

and no sings of powder diffraction were observed. It was found

out that the non-irradiated crystal 1b already contained 16.6% of

the product. Attempts to determine its structure as of the pure

reactant caused a significant decrease in the quality of the model

(for instance, R1 increased from 0.076 to 0.085).

The X-ray data were collected by means of a CCD diffrac-

tometer.12 The structures were solved using SHELXS97.13 The

first atoms of the minor component were located in difference-

Fourier maps. The remaining atoms of this component were

positioned geometrically. The structures were refined by means

of SHELXL97.13 In general, the major component was refined

anisotropically and the minor component isotropically.

However, for crystal 1a, benzene rings C1/ C6, C8/ C13 and

C28 / C33 were treated isotropically and sometimes as rigid

rotating groups. In the case of the structures of 40.2 and 46.2%

conversion for crystal 1b, rings C1/C6, C8/C13 and C28/

C33 were also treated in such a manner. For the remaining

structures, i.e. for 16.6, 20.5 and 25.5% conversion, rings C8 /

C13 and C28 / C33 were always refined isotropically as rigid

rotating groups. Owing to a reactant–product disorder, the

following weak restraints from SHELXL9713 were applied:

DFIX, DANG and SIMU. DFIX and DANG restrained bond

lengths and valence angles to target values. The target values

were taken from the structures of the pure reactant and the pure

product of crystal 1a. SIMU restrained displacement parameters

of atoms of the products.

In general, hydrogen atoms in crystals 1a and 1b were posi-

tioned geometrically and treated as riding. However, in the pure

reactant and the pure product crystals, hydrogen atoms of the

–NH and –OH groups were not constrained. The hydrogen atom

of the –OH group was omitted in partly reacted crystals.

Hydrogen atoms of the –OCH3 group were located in a differ-

ence Fourier map and refined as part of a rigid rotating group,

however, in the case of the minor component in crystal 1b they

were omitted for the first three structures and also for molecule C

in the structure of 40.2% conversion.

17.4%P for A 23.6%P for B (1b) 32.7%P for A 59.6%P for B (1b)

C33H39NO4 C33H39NO4

513.65 513.65c Orthorhombic Orthorhombic

Pna21 Pna2122.121 (4) 22.199 (4)9.3817 (18) 9.3700 (17)28.942 (5) 29.003 (6)6006.4 (19) 6033 (2)8 81.136 1.1310.07 0.07299(2) 299(2)27 966 28 1795379 53990.051 0.0580.075, 0.191 0.091, 0.2420.21, �0.24 0.20, �0.18

CrystEngComm, 2011, 13, 4332–4338 | 4333

Fig. 1 ORTEP22 view of the product molecule (red) superimposed on

the reactant molecule (black) for crystal 1a containing 55.6% of the

product. Displacement ellipsoids are drawn at the 10% probability level.

Hydrogen atoms are omitted for clarity. Only the reactant atoms and two

product atoms forming the new bond are labelled. The labelling scheme is

the same for crystal 1b.

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The selected crystal data for five structures are presented in

Table 2, and for the remaining structures as ESI‡. The complete

data can be found in the cif files (ESI‡).

Results and discussion

The S-enantiomer of 1 crystallizes in the space group P212121with one molecule in the asymmetric unit, crystal 1a. The racemic

mixture crystallizes in the space group Pna21 with two symmet-

rically independent molecules, A and B, crystal 1b. The attempt

to determine the structure of the racemic mixture in the centro-

symmetric space group was unsuccessful.

Geometrical requirements for the photochemical reaction

In molecules of 1, there are two o-isopropyl groups in the

g-position from the carbonyl group and both of them can

potentially take part in the Norrish-Yang reaction. Nevertheless,

the structure determination revealed that only one of them is

reactive.

In general, the Norrish-Yang reaction occurs when geomet-

rical parameters involving a carbonyl group and atoms of

a gC–gH bond have appropriate values.14,15 These geometrical

parameters are presented in Scheme 1c. Their ideal values and

the average literature values together with the ranges for pho-

toactive compounds undergoing the Norrish-Yang reaction are

given in Table 3. The data for crystals 1a and 1b are also

included. As can be seen, the values of d, u and D for one o-

isopropyl group in crystal 1a are out of the ranges for photo-

active compounds but the values for the opposite o-isopropyl

group indicate that it can take part in the reaction. This is

consistent with the observations made during the structure

determinations. In the case of crystal 1b the situation is not so

straightforward. The values of the geometrical parameters for

molecules A and B are not definitely better for the reactive o-

isopropyl group. Moreover, some of them are even worse than

for the inert o-isopropyl group. However, the values of d, D, D

and Q for both o-isopropyl groups in molecules A and B are

within the literature ranges and only u for the inert o-isopropyl

group is slightly out of the range found for compounds

undergoing the Norrish-Yang reaction.

ortho-Alkylphenyl ketones can also undergo a d-abstraction.21

However, this is not the case for crystals 1a and 1b. The smallest

distance between the carbonyl O and d-H atoms, d, is 3.66 and

Table 3 Values of the geometrical parameters influencing the Norrish-Yang

d/�A D/�A

Ideal values <2.7 <3.4Average literature valuesa 2.64 (8) 3.00 (9)Literature rangeb 2.49–2.95 2.82–3.121ac 2.70 2.901 (6)

3.00 2.915 (6)1b, molecule Ac 2.92 2.999 (10)

2.84 2.877 (10)1b, molecule Bc 2.83 2.998 (11)

2.83 2.884 (12)

a d, u, D and Q for 54 aromatic ketones undergoing the Norrish-Yang14 andQ5,7,8,14,16–20 and 16 compounds for D.5,7,8,18,20 c For the non-irradiated crystalo-isopropyl groups, respectively.

4334 | CrystEngComm, 2011, 13, 4332–4338

3.84 �A for molecules A and B in crystal 1b, respectively and 3.05�A for crystal 1a (values for the non-irradiated crystals). These

values, and also the values calculated for the irradiated crystals,

are too large for the d-abstraction reaction to occur.

Fig. 1 presents the reactant and the product molecules super-

imposed on each other for the partly reacted crystal 1a. As can be

seen, the overall shape of both molecules is similar. The same is

true for crystal 1b.

Monitoring cell parameters

Variations in cell parameters with the time of UV-vis irradiation

are one of symptoms of photochemical reactions in crystals.

Fig. 2 presents variations in the cell parameters with the reaction

progress for crystal 1a. As can be seen, the parameters change in

a different way. The changes are 1.4, 6.5 and 6.3% for a, b and c,

respectively. For c this is almost 2 �A. The variations in param-

eters a, b and c influence the variations in the cell volume (Fig. 2).

The total change in V is 1.2%. The extent of this change is typical

for intramolecular photochemical reactions in crystals.5,7–9

Fig. 3 presents the relevant data for crystal 1b. The largest

changes for a, b, c and V are 0.8, 1.4, 1.6 and 0.8%, respec-

tively. As can be noticed by comparison of Fig. 2 and 3, there is

a difference between the character of the changes in a, b and c

for crystal 1a and crystal 1b. This is connected with packing

and orientation of molecules in the crystals but also with the

reaction in crystals

u/� D/� Q/�

0 90–120 18054 (10) 82 (8) 116 (3)50.8–83.1 52.9–88.0 112.0–128.076.9 61.5 117.786.4 49.4 119.478.1 58.2 120.384.1 54.5 119.975.7 62.1 122.483.7 55.1 121.2

D for 53 structures.15 b On the grounds of 39 compounds for d, u, D ands; the first and the second lines are for the photoreactive and photoinert

This journal is ª The Royal Society of Chemistry 2011

Fig. 2 Percentage variations in the cell parameters and the cell volume

for crystal 1a with the content of the product in the crystal. Standard

uncertainties are 0.0012–0.002, 0.0016–0.003, 0.004–0.007 �A and 0.7–1.3�A3 for a, b, c and V, respectively.

Fig. 3 Percentage variations in the cell parameters and the cell volume

for crystal 1b with time of irradiation of the crystal. Standard uncer-

tainties are 0.003–0.004, 0.0013–0.0019, 0.005–0.006�A and 1.9–2.0 �A3 for

a, b, c and V, respectively.

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additional factor described next. Fig. 2 and 3 show that the cell

volume for 1b increases linearly until ca. 40% reaction progress

(20 min of irradiation is equivalent to 46.2% product content in

this crystal) but for 1a it is statistically constant in the same

range. In the case of crystal 1b the changes are not the result of

the photochemical reaction only but they are also the result of

a distinct decrease in crystal diffracting power under the

influence of UV radiation. The phenomenon of this type,

brought about by UV radiation, was rarely monitored.23

However, there are many papers concerning the influence of

X-rays and especially synchrotron radiation on a loss of

translational order. A decrease in crystal diffracting power was

usually accompanied by a linear increase in the cell volume and

was attributed to an increase in the number of defects.23–27

Taking into account the above statements, we can say that the

observed changes in the cell parameters and the cell volume of

crystal 1b are the result of two effects: the photochemical

reaction and the loss of translational order. Moreover, the

constancy of the cell volume of 1a in the first stage of

the reaction (until ca. 40% reaction progress) indicates that the

initial increase in the cell volume for crystal 1b is the result of

the loss of translational order, at least in significant part.

In the scientific literature there are known compounds

undergoing one type of a photochemical reaction in crystals and

for which variations in the cell volume have different charac-

ters.5,7,28,29 However, to our knowledge there are no papers

comparing paths of changes in a cell volume for two different

crystal forms of a single compound.

This journal is ª The Royal Society of Chemistry 2011

Monitoring the content of the product in the crystals

The content of the product in the partly reacted crystals was

found out from the refined site occupations factors, SOFs.

Fig. 4a presents the variations in the product content with the

time of irradiation of crystal 1a. The relationship is not linear and

shows that the reaction is fast at the beginning and slows down in

the final stages. Relationships of this type were also observed in

the case of other photochemical reactions.2,3,5–8,30,31

Crystal 1b is very interesting owing to two symmetrically

independent molecules, A and B. The changes in the content of

the product obtained frommolecules A and frommolecules B are

presented in Fig. 4b. As can be seen, molecules A and B react at

different rates. For instance, after 20 min of irradiation, the

progress of the reaction is ca. 33% and 60% for A and B,

respectively. This observation can be rationalized by means of

the concept of free space. It is known that too small free space

disables an occurrence of a photochemical reaction.11,20,32–34 For

the non-irradiated crystal 1b the volume of the free space is by

10 �A3 larger for molecule B than for molecule A. Moreover, the

values of the geometrical parameters deciding about the Norrish-

Yang reaction are better in the case of molecule B than molecule

A (see Table 3).

It is known from the scientific literature that molecules change

slightly their shape during reactions in crystals.5–9 Owing to this it

can be expected that the geometrical parameters and also the free

space will change with the progress of the photochemical reac-

tion in crystal 1b. The relevant relationships for the distance

CrystEngComm, 2011, 13, 4332–4338 | 4335

Fig. 4 Variations in (a) the product content in crystal 1a, (b) the product

content for molecules A (squares) and B (triangles) in crystal 1b and (c)

the distance between the directly reacting atoms in the reactant molecules

A (squares) and B (triangles) in crystal 1b and the difference between the

volume of the free space for molecules B and A (red circles) in crystal 1b

with time of irradiation of the crystals. DVfree ¼ Vfree(B) � Vfree(A) was

calculated by means of PLATON35 (the instruction ‘‘calc void’’ for the

structures with several atoms omitted near the reaction centre of mole-

cules A and B, respectively). Standard uncertainties are 0.6–1.2, 0.7–1.1

and 0.8–1.1% for (a) and (b) and 0.010–0.02 and 0.011–0.03 �A for (c) (for

the D distance in molecules A and B).

Fig. 5 Projection of the molecules of (a) the reactant for 0% (black), 15%

(red), 28.7% (green), 55.6% (blue) and 81.2% (violet) reaction progress

and (b) the product for 15% (red), 28.7% (green), 55.6% (blue), 81.2%

(violet) and 100% (black) reaction progress for crystal 1a onto the plane

of the middle ring.

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between the directly reacting atoms of the reactant molecule, D,

and for the volume of the free space, DVfree, are presented in

Fig. 4c. As can be seen, the volume of the free space, Vfree, is not

always better for the more reactive molecule B. However, in such

a situation the intramolecular geometrical parameters, and

among them D, are better. Generally speaking, the rate of the

photochemical reaction is described by the current values of the

parameters. Taking into account the literature data it can be said

4336 | CrystEngComm, 2011, 13, 4332–4338

that also a kind of a product is the result of current (and not only

initial) values of some parameters.6

In the scientific literature there are known compounds of

different reactivity of symmetrically independent molecules36–40

but there is also known an example where two symmetrically

independent molecules react at the same rate.41 The rates of

reactions were usually determined by means of HPLC and very

seldom on the grounds of structures of partly reacted crystals.36–40

Differences in reactivity were attributed to differences in

a volume of a reaction cavity36,39,40 and/or differences in

geometrical parameters of molecules.37,38 However, the volume

of the reaction cavity was monitored very seldom.40

Monitoring the shape and orientation of molecules

The literature data show that molecules of a reactant and

a product slightly change their shape with reaction progress in

crystals.5–9 Such observation was also made in the case of crystals

1a and 1b. The relevant results for molecules in crystal 1a are

shown in Fig. 5. For molecules in crystal 1b the picture is very

similar.

This journal is ª The Royal Society of Chemistry 2011

Fig. 6 Variations in the angle between the C28/ C33 benzene ring and

the bc plane for molecule A (black squares) and the ab plane for molecule

B (red triangles) with the reaction progress in crystal 1b. Standard

uncertainties are 0.7–1.0 and 0.7–0.8�, respectively.

Fig. 7 Variations in the D/A distance for the N–H/O hydrogen bond

formed between the reactant (black) and the product (red) molecules in

crystal 1a. Standard uncertainties are 0.004–0.12 and 0.005–0.02 �A and

the biggest changes are 0.086(13) and 0.14(3) �A, respectively (statistically

significant on the 3s level, i.e. with 99.7% probability).

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We also monitored the variations in the orientation of mole-

cules and molecular fragments in the unit cells for crystals 1a and

1b. These variations are smooth for most of molecular fragments.

In the case of crystal 1a the smallest change was observed for the

C1 / C6 benzene ring of the reactant (ca. 10�) and the biggest

one for the –COOCH3 group of the reactant (ca. 16�). For thereactant molecules in crystal 1b the biggest changes were found

for the C28/ C33 benzene ring and the –COOCH3 group (ca. 5

and 4� for molecule A, respectively and 8 and 10� for molecule B,

respectively). For the product molecules C and D (formed from

molecules A and B, respectively) the biggest changes were

observed for C1/ C6, C28/ C33 and –COOCH3 (ca. 4, 8 and

11� for molecule C, respectively and 16, 4 and 16� for molecule D,

respectively). Fig. 6 presents the selected relationships. The

changes in the orientation of molecular fragments increase in the

following order: A < B z C < D. The largest changes reported

for intramolecular reactions are ca. 20�.5–9

Monitoring hydrogen bonds

In crystals 1a and 1b there are strong N–H/O bonds. Both the

reactant and the product take part in such bonds. The geometry

of the hydrogen bonds is given in Table 4. In crystal 1a the

hydrogen bonds form chains along the a axis and in crystal 1b—

Table 4 Geometry of the hydrogen bonds in crystals 1a and 1b (�A,�).R ¼ reactant, P ¼ producta

Crystal,conversion Bond D/A D–H H/A :DHA

1a, 0% N1R–HN1R/O2Ri

2.931(4) 0.91(4) 2.06(4) 160(3)

1a, 100% N1P–HN1P/O2Pi

2.860(5) 0.85(5) 2.04(5) 162(5)

1b, 16.6(6)% N1A–H1A/O2Bii

2.933(12) 0.86 2.09 166

N1B–H1B/O2A

2.919(12) 0.86 2.08 164

a Symmetry codes: (i) x�1/2, �y + 1/2, �z; (ii) x, 1 + y, z.

This journal is ª The Royal Society of Chemistry 2011

ribbons. For the reason that with the reaction progress the whole

crystal structure changes, the hydrogen bonds also change their

geometry. In the case of crystals 1a and 1b they are not disrupted.

Most of the relationships between the geometry of the hydrogen

bonds and the reaction progress are not smooth but even then the

total changes are statistically significant. Fig. 7 presents the

selected relationships for crystal 1a.

Conclusions

An increase in the volume of a unit cell brought about by UV

irradiation of crystals can be a symptom of not only a photo-

chemical reaction but also a loss of translational order.

The rate of a photochemical reaction and also the kind of

products are affected by current crystal structures. The analysis

of structures of partly reacted crystals can be helpful in under-

standing photochemical reactivity of compounds in crystals.

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