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research papers Acta Crystallographica Section B Structural Science

Acta Crystallographica Section B Structural

Science

ISSN0108-7681CSDSymmetry:the definitive database of point-group and space-group symmetry relationships in small-molecule crystal structures

Jing Wen Yao,a Jason C.Cole,a Elna Pidcock,a Frank H.Allen,a Judith A.K.Howard b and

W.D.Samuel Motherwell a*

a Cambridge Crystallographic Data Centre,12 Union Road,Cambridge CB21EZ,UK,and

b Department of Chemistry,University of Durham,South Road,Durham DH13LE,UK

Correspondence e-mail:

motherwell@https://www.wendangku.net/doc/de7320456.html,

applied to ca.200000entries from the Cambridge Structural

Database(CSD).For each molecule,the perceived point

group,together with crystallographic properties such as space

group,occupied Wyckoff positions and number of residues in

the asymmetric unit,have been placed in a relational database, CSDSymmetry,using Microsoft Access software.Database

queries can be constructed easily to?nd occurrences of any

combination of molecular or crystallographic attributes,and

thereby to answer questions on relative distributions.Some

typical example queries are given.The inclusion of CSD

reference codes enables direct visualization of search results

using the Cambridge Crystallographic Data Centre's three-

dimensional structure visualizer,Mercury.

Received25February2002

Accepted15April2002

1.Introduction

The question of why molecules pack to form particular crystal

structures is of interest to many?elds in science.For example,

in crystal structure prediction and crystal engineering,estab-

lishing reliable correlations between molecular symmetry and

crystallographic symmetry could prove invaluable(see e.g.

Filippini&Gavezzotti,1992),while in the design and synthesis

of materials having non-linear optical properties,non-centrosymmetric space groups are required,and space-group

control at the molecular level is highly desirable.

One approach to understanding the relationships between

molecular and crystallographic symmetry is to gather statistics

from existing crystal structures.Several published studies have

considered molecular symmetry properties along with space-

group frequency distributions(see,e.g.Scaringe,1991;Zorky

et al.,1993;Wilson,1993;Brock&Dunitz,1994;Belsky et al.,

1995;Steiner,2000and references therein).In some of these

studies,the distribution of space groups was examined with

respect to the number of molecules per asymmetric unit(Z H),a

property that is loosely correlated with molecular symmetry.

Others have provided classi?cations based on the occupied

Wyckoff positions across a wide range of space groups,de?ned

as structure classes by Belsky et al.(1995).These publications

have led to the formulation of some general rules of crystal

packing(see Brock&Dunitz,1994),such as`mirror planes in

mirror symmetric space groups are always occupied'.Other

studies(Brock&Duncan,1994;Lloyd&Brock,1997),have

focused on a set of molecules with a common structural motif

and have examined their distribution through space groups

and the occupancy of special positions in these space groups.

The Cambridge Structural Database(CSD)(Allen,2002;

Bruno et al.,2002)has played an integral role in many of the

studies cited.The CSD is the de?nitive database of published

organic and metal-organic structures determined by X-ray and

neutron diffraction techniques.The information contained within the database is primarily concerned with recording molecular structure as three-dimensional atomic coordinates with respect to a crystallographic unit cell and the space-group setting.However,it is recognized that molecular symmetry information is inherently present in the database and could be extracted;hence statistical analysis of molecular and crystal-lographic symmetry relationships could be performed using the entire database.A recent publication(Cole et al.,2001) described a molecular-symmetry perception algorithm and its implementation in RPluto(Motherwell et al.,1999).

The present paper describes the application of this algo-rithm to approximately200000molecules retrieved from the CSD.The resulting data,together with crystallographic properties such as space group,symmetry of occupied Wyckoff positions(special or general positions)and Z H,have been collated and entered into a relational database.Inter-rogation of this new database is possible with user-de?ned queries,and hence the database provides an extremelyˉexible source of symmetry-related information.Since the results of a query can be retrieved as a list of CSD reference codes (Refcodes),the corresponding molecules can be viewed easily in the Cambridge Crystallographic Data Centre's(CCDC's) three-dimensional structure visualizer,Mercury(Taylor& Macrae,2001;Bruno et al.,2002).1

2.Methodology

2.1.Choice of dataset

The April2000release of the CSD,version5.19,containing 215403structures,was used to create a dataset from which to extract molecular and crystallographic information.The Quest3D program(Cambridge Crystallographic Data Centre, 1994)was used to select the dataset.Only those structures having three-dimensional coordinates were considered,and structures were further excluded if they(a)contained dis-ordered molecules or ions,2(b)were polymeric(catena structures)or(c)contained connectivity matching errors,i.e.a 1:1mapping of the chemical and crystallographic connectiv-ities had been unsuccessful.Only one member of each Refcode family(i.e.structures having the same six-letter code) was retained to avoid bias in statistical analysis of the symmetry database.Crystal structures that represent different conformations of the same molecule are present within the CSD.If these crystal structures have the same six-letter Refcode,only one example will be retained within the symmetry database.A?le of the surviving structures was created in the CCDC's FDAT format,which was then used for the extraction of symmetry and crystallographic information.2.2.Molecular symmetry perception

A method for detecting approximate molecular symmetry in crystal structures has been developed previously(Cole et al., 2001).In brief,the symmetry elements for each molecule are identi?ed within certain geometrical tolerances,and the combination of the symmetry elements present for the mole-cule allows its assignment to a point group.The perception of molecular symmetry is performed using only the atomic coordinates,and no information regarding any coincident crystallographic symmetry is included in the analysis.

The molecular perception algorithm was implemented in RPluto through the msym command.RPluto,when provided with a list of retrieved CSD entries in FDAT format,can perform molecular symmetry identi?cation as a batch process; the molecular point group,the Refcode,the number of resi-dues,the space group(Hermann±Mauguin symbol),Z,Z H and the number of matrices used to detect the molecular symmetry are returned for each entry.The FDAT?les of the retrieved dataset were submitted to the symmetry detection process and the point groups of198335molecules were assigned.For this analysis of molecular symmetry in the CSD,the default geometric tolerances used on distance and angle(Cole et al., 2001)were0.1Aêand5.0 ,respectively.Molecules that contain fewer than three non-H atoms were not treated by the algo-rithm.The CSD bond-type code(a code that enumerates types of bond,i.e.single,double)was ignored and H atoms were disregarded for the purposes of symmetry detection,the implications of which are discussed in the next section.

2.3.Chemical considerations

Firstly it should be reiterated that structures tagged with the CSD disorderˉag have not been included in the processed dataset.However,it is known that there are structures contained in the CSD that have been re?ned without proper resolution of disorder.Such structures often have highly variable geometry in regions of unresolved disorder.These structures are included in the processed dataset and may result in anomalous symmetry assignments.

As was mentioned above,the symmetry perception algo-rithm makes use of geometric tolerances for distance and angle.If the tolerances are set too tightly then very little symmetry will be perceived within a molecule.Conversely,if the tolerances are very large then symmetry will be deter-mined to be present where it is not.The tolerances used in the analysis of this dataset were found to represent a reasonable compromise between`missed symmetry'and`false symmetry'. In the current implementation of the symmetry perception algorithm,the CSD bond-type code is not considered.In some cases,this will lead to a`false'point-group assignment.For example,8-methoxynaphthalene-1-carboxylic acid[CSD entry MXNACX(Schweizer et al.,1978)]is identi?ed as having a mirror plane even though the carboxyl group is oriented perpendicular to the plane of the aromatic rings:the CDO bond is not distinguished from the C O bond and the H atom is ignored.This type of assignment will occur with groups of the type BDA B(where A may equal B)(Fig.1).If there is a

1RPluto and Mercury can be downloaded for non-commercial research purposes from https://www.wendangku.net/doc/de7320456.html,/prods.

2Disorder is indicated as being present within the CSD if there is disorder of any atom sites,including solvent molecules.

symmetry element that passes through the central atom A (for example,a C 2axis or a mirror plane),in the absence of bond-type information,the geometric tolerance of 0.1A

êis such that A B is not distinguishable from A DB .

To illustrate this point further,molecules (or fragments of molecules)found in the CSD that contain a B DA B group are shown in Fig.1along with the perceived symmetry.It can be seen that the decision to disregard bond-type information is justi?ed in the cases of the nitrate ion and,in some instances,the carboxylate ion.However,in the examples illustrated by the molecules GOTQEL (Antorrena et al .,1999)and DIVRIJ (Watson et al .,1986)(Fig.1),exclusion of bond-type infor-mation leads to false symmetry assignments.To establish how frequently the presence of a B DA B group perturbed the point-group assignment,a sample of 200molecules was randomly selected from the CSD and the molecular point group determined by visual inspection was compared with the point group detected by the msym command implemented in RPluto .Only ?ve examples were found where the formal point-group assignment was false.

Disregarding H atoms can lead to errors in molecular point-group assignment.For example,aqua-hydroxy-("2-3,5-bis{bis[2-(diethylamino)ethyl]aminomethyl}pyrazole)-di-nick-el(II)bis(tetraphenylborate)[Meyer et al.(1999),CSD entry HOCKEP],a binuclear nickel complex,is determined to have C 2(2)symmetry when H atoms are ignored,thus identifying the two Ni atoms as being symmetry-equivalent.However,this symmetry is broken when H atoms are taken into considera-tion,as one nickel is coordinated to a water molecule and the other nickel is bound to a hydroxide.From the subset of 200structures randomly selected from the CSD (see above),only

three molecules were identi?ed as belonging to a point group of lower symmetry once the H atoms were taken into consideration.

2.4.Supplementing the dataset

It was thought to be useful to include information other than that generated by RPluto ,and hence a local Fortran program was written to read the retrieved FDAT ?les and supplement the basic table.The further information extracted was space-group number,the number of atoms and the number of H atoms per molecule (or ion),the R -factor of the crystal structure,whether a metal atom is present in the molecule,the SIGCC ˉag,3and the symmetry of the occupied Wyckoff position(s).Unfortunately the CSD does not contain an easily-extractable marker or ˉag that details whether a structure is enantiomerically pure or not.Thus the dataset does not include any information regarding the chirality of the molecules or the optical activity of the structures.

The algorithm that determines the symmetry of the occu-pied Wyckoff position(s)uses symmetry-related atoms (Satoms )stored within the CSD entry:these are atoms generated by space-group symmetry that,together with atoms from the asymmetric unit,complete the chemical structure of a molecule or ion.Their coordinates and atom labels can be used to recognize the space-group symmetry operators that were used to generate the complete molecule.Additional checks are performed for symmetry operators that are known to be present in the space group but which are not detected through the presence of Satoms :for example,if a planar molecule is lying on a mirror plane.In these situations,the presence of the symmetry operator is tested by using the full coordinate set (Cole,1995).It is known that a molecule can occupy a Wyckoff position of lower symmetry than its mole-cular point group;an example is illustrated in Fig.2,and the choice of Wyckoff position occupied by a molecule may be of signi?cant interest.

The additional data items were appended to the data generated by RPluto to complete the ?nal table,exempli?ed for a selection of entries in Table 1.It should be noted that this table is assembled on a molecular basis;it contains an entry for each crystallographically unique molecule or ion in each crystal structure that has more than two non-H atoms.This table,which summarizes crystallographic information as well as molecular and crystal symmetry properties,is a very rich source of information,and the size of the dataset from which it was generated allows a thorough exploration of the possible relationships between crystallographic and molecular symmetry.However,in ˉat-?le format,the information is not readily accessible.

2.5.Creation of the database

In order to interrogate these data with a wide range of queries,the table has been placed in a relational

database

Figure 1

(a )Illustration of a B DA B group with a symmetry element that will not be found with the RPluto symmetry detection algorithm with the

default tolerances of 0.1A

êand 5.0 .There will be other functional groups that behave similarly,not illustrated here.(b )A selection of molecules or fragments of molecules taken from the CSD,drawn using the CSD bond-type code convention.From left to right para -bromotetraˉuorophenyl-1,2,3,5-dithiadiazolyl radical (Y =Br and X =F,CSD entry GOTQEL),nitrate ion,carboxylate group and exo -3,4,5-trithiatetracyclo-(5.5.1.02,6.08,12)tridec-10-ene,(CSD entry DIVRIJ).The perceived molecular symmetry is appended to each structure.The B DA B group is highlighted with a rectangle,and a symmetry element C 2(2)or C s (m ),which might not be found by the symmetry detection algorithm at the default tolerances,is also shown.

SIGCC categorizes the mean structural precision for CDC bonds in each structure on a scale of 1±4;1indicates a high degree of precision and 4indicates a low precision.

using Microsoft Access and supplemented with additional tables.The ?le containing the CSD-derived information was manipulated so that it could be read into Access as a plain-text ?le and as a single table,denoted as CSDSymmetry .Upon entry into Access ,it was found that there were 106entries for which the occupied Wyckoff position had not been deter-mined.These records were removed from the database,and thus the database contains information for 198229molecules.

The database has been supplemented with two further

tables.A descriptive table for molecular point groups (PointGroupInfo )contains 38common point groups (including non-crystallographic point groups such as C 5)and their component symmetry elements.Similarly,a descriptive table (SpaceGroupInfo )contains the symmetry of the Wyckoff positions that characterize each of the 230space groups,and the table SymmetryOps contains the symmetry operators for each of the 230space groups.The CSDSymmetry table is related to the auxiliary table PointGroupInfo through the ?eld `point group'and to the tables SpaceGroupInfo and SymmetryOps through the ?eld `space group number'.The inclusion of the descriptive tables allows a wide range of searches to be performed;for example,all space groups that contain a Wyckoff position with symmetry m (C s ),or all point groups containing a mirror plane,can be located easily.

The database has facilities that permit analysis of the full table contents or of subsets resulting from queries.For example,results can be sorted in descending order of occur-rence,grouped,counted or manipulated with user-de?ned equations.An illustrative example query is:`Find the point group with the greatest number of molecules in the space group P21/c,and present the result as a percentage of the total number of molecules in the database.'This requires that all molecules found in crystal structures belonging to space group P 21/c are collated by their point group,the number of molecules per point group is counted,and the counts are sorted in descending order (the answer is C 1,see Fig.3).The number of molecules of C 1(1)symmetry found in P 21/c is then divided by the total number of molecules in the database via a user-de?ned equation to give the answer of 11.4%.Further,those molecules that belong to point group C 1and space group P 21/c can be gathered as a Refcode list.The three-dimensional structure visualizer Mercury 1.0(Taylor &Macrae,2001;Bruno et al.,2002)accepts a list of Refcodes as input (from a .gcd ?le)so that the results of the query can be viewed.In suitable cases,Microsoft Excel can also be used to present query results in graphical form (Fig.3).

Once the CSDSymmetry database has been downloaded,it can be customized by the user in many ways:additional ?elds can be added to the existing tables or whole new tables can be added to the database.The results from a query can be saved and this subset,a user-de?ned database,can be used as a basis

Table 1

Six entries in the table constructed by the systematic extraction of symmetry-related information from the CSD.

This table forms the basis of the relational database CSDSymmetry .The last column in the table (Mpres)is a ˉag that has the value of 1if a metal atom is present and 0otherwise.

Refcode Residue number Point group Space group Z Number of atoms R -factor HM Wyckoff position Occupied Wyckoff position Space-group number Z H

Number of H atoms SIGCC Mpres PIZYA Y 1C (1)Pna 214410.0571C 13312330PIZYEC 1C (1)P 21/n 4440.0461C 11412010PIZYIG 1C (s )P 212400.0351C 1412020PIZYOM 1C (1)P "12870.0271C 1213431PIZZAZ 1C (i )P 21/n 4130.048à1Ci 141001PIZZAZ

C (1)

P 21/n

0.048

C 1

Figure 2

(a )Extract from CSDSymmetry showing the database entry for CSD Refcode YIRPOE (Childs et al.,1994).There are three molecules listed for this crystal structure.(b )Crystal structure (Childs et al.,1994)of 2-methyl-5-ethoxymethyl-1,3-dioxan-2-ylium hexachloroantimony (Ref-code YIRPOE).The molecules are coloured by symmetry equivalence;the octahedral SbCl 6àions occupy Wyckoff positions of C 2(2)(green)

and i "1

(blue)symmetry.

for more selective queries.Importantly,the results of any chemical or substructural search in ConQuest (Bruno et al.,2002)can be intersected with the CSDSymmetry database.Thus,a ConQuest search for the presence of a carboxylic acid group will return a list of Refcodes that can be entered into the CSDSymmetry database.Where a Refcode from the ConQuest search matches a Refcode in the CSDSymmetry database,crystallographic and molecular symmetry information can be extracted and analyzed.2.6.Validation

Independent examinations of the CSD by several authors over the last few years have yielded symmetry-related corre-lations and their results provide suitable queries with which to perform an initial validation of CSDSymmetry .Brock &Dunitz (1994)have outlined a number of correlations observed between space group and molecular symmetry.For example,the authors state `inversion centres are favourable'and go on to show that centrosymmetric molecules often occupy inversion centres in space groups.A search of CSDSymmetry (with the criterion that the number of residues must not be greater than one)reveals that there are 18008molecules that belong to point groups containing the

symmetry element i "1

.Of these,17152molecules (95.2%)crystallize in space groups containing a Wyckoff position of

symmetry i "1

,and 15156molecules (88.4%)utilize the crystallographic inversion centre.

Brock &Dunitz (1994)also state `groups with threefold axes do not usually occur unless axes are located within molecules of the appropriate symmetry'.This conclusion was drawn from space groups having special positions of C 3symmetry only.A search of CSDSymmetry revealed 2351crystal structures where the space group contains a threefold axis;the presence of other special positions was permitted.There are 1158structures with an occupied Wyckoff position

of symmetry C 3(3),and all of these special positions are occupied by molecules belonging to a point group that contains a threefold rotation axis.Brock &Dunitz (1994)also note `twofold rotation axes are sometimes occupied and sometimes not'.Of the 198229molecules within the database,24332(12.3%)are found within space groups that contain a twofold axis.Of these,8760molecules (36.0%)are located on the twofold axis.

Thus,concepts that were established from relatively small and hand-edited datasets,and which have been corroborated by a number of authors,are fully supported by the more extensive dataset contained within the CSDSymmetry data-base.

3.Example applications of CSDSymmetry

Some further examples of the use of CSDSymmetry have been outlined below to exhibit the utility of the database.These examples are meant to be illustrative rather than an exhaus-tive demonstration of the types of queries that can be put to the database.Detailed statistics and specialized studies of symmetry relationships determined using CSDSymmetry will be presented in later publications.

Example 1:What space groups are represented by molecules of common point groups ?In the ?eld of crystal structure prediction,it may be informative to know what impact the molecular point group has on the choice of space group (Belsky et al.,1995;Scaringe,1991).For the entire database,the ?ve most populated space groups are (in descending

order)P 21/c ,P "1,

P 212121,P 21and C 2/c .These ?ve space groups account for 81.7%of the structures contained within the database;P 21/c (space-group number 14)alone accounts for 36.6%of the database.For a molecule of the point group C 2(2),4the ten most populated space groups are (in

descending order)C 2/c ,P 21/c ,P "1,

Pbcn ,P 212121,P 21,P 2/c ,Pbca ,C 2and P 21212.Of these space groups,C 2/c ,Pbcn ,P 2/c ,C 2and P 21212contain a Wyckoff position of symmetry C 2(2),and approximately 92%of the molecules are found residing on such an axis.The three most populated of these space groups (C 2/c ,Pbcn and P 2/c )also contain a centre of inver-sion.The next three space groups that contain a twofold axis are C 2,P 21212and Fdd 2;these space groups only contain the C 2Wyckoff position and no centre of inversion.Why do molecules of C 2symmetry seem to prefer space groups with a twofold axis and an inversion centre?Wilson (1993)stated that if a special position of a crystal structure is occupied then the packing of the crystal is `F F F as if the symmetry of the space group were degraded to that of a subgroup lacking the molecular symmetry in question'.The maximal non-isomorphic subgroups (with the C 2axis removed)of C 2/c ,

Pbcn and P 2/c include P "1

and P 21/c ,the two most populated space groups of the entire database.The maximal non-isomorphic subgroups generated on removal of the C 2

axis

Figure 3

Histogram plotted from data retrieved using CSDSymmetry showing the 11most prevalent molecular point groups and their frequencies in crystal structures belonging to the space group P 21/c .

The database does not include information on the optical activity of the compounds and so no distinction is made between enantiomerically pure C 2structures and racemic C 2structures.

from C2,P21212and Fdd2are P1,P21and Cc.These space groups are less common than P21/c and P"1.Therefore,the observation that C2molecules prefer space groups that contain Wyckoff positions of C2(2)and i "1 symmetry is a consequence of`accidental symmetry':really it is a preference for the packing motif found in the space groups P21/c or P"1 that is expressed.In other words,a molecule with C2symmetry ?nds favourable close packing in space group P21/c.Main-taining the same packing motif,but with its twofold axis aligned with a C2special position,the space group becomes C2/c with a half-molecule as the asymmetric unit.

For a molecule of point group C s(m),the?ve most popu-lated space groups in descending order are P21/c,P"1,Pnma, P212121and C2/c.The promotion of the space group Pnma from the tenth most populated in the entire database to the third most populated for molecules of C s(m)symmetry can be explained by the following:Pnma is the only space group of the top ten most populated space groups that contains a Wyckoff position of symmetry C s(m)in the database.Of the 1315molecules of C s(m)symmetry that are found in Pnma, only42(3.2%)do not crystallize on the special positions C s(m)or C2v(mm).Again it is interesting to note that a maximal non-isomorphic subgroup of Pnma(with the inver-sion centre removed)is P212121,the third most populated space group of the CSDSymmetry database.

Example2:Are the most popular space groups the same for metal-containing molecules and purely organic molecules?The ?ve most popular space groups for CSDSymmetry entries that do not contain a metal atom are the same as for the entire database,i.e.P21/c,P"1,P212121,P21and C2/c in descending order.If the criterion that a metal atom is present is imposed5 and the query is performed again,the?ve most popular space groups,in descending order,are P21/c,P"1,C2/c,P212121and Pbca.Therefore,in the metal-containing dataset,the space group P21(no special positions)has become less prominent and Pbca,a space group with an inversion centre,has been promoted.A histogram showing the distributions of metal-containing and non-metal-containing molecules is shown in Fig.4.Searching CSDSymmetry reveals that of the112198 molecules for which the metal-presentˉag is equal to one,half the dataset(56610records,50.4%)are of C1(1)point-group symmetry.This?gure is closer to70%for the organic dataset (86031structures,59216belong to point group C1).Conse-quently,approximately half of the metal-containing dataset (48.6%)belong to point groups with some symmetry:at least an inversion centre and/or a twofold rotation axis and/or a mirror plane.For the organic dataset,substantially less than half(only30.8%)of molecules belong to point groups with symmetry.Thus,in broad terms,it can be concluded that metal-containing molecules exhibit more symmetry than purely organic molecules.The popularity of space group Pbca in the metal-containing dataset is perhaps an indication that symmetric molecules have a preference for space groups with symmetry.

Example3:Do high symmetry space groups prefer highly symmetric molecules?There are1420molecules with more than?ve atoms found within space groups that contain both of the special positions of symmetry C s(m)and C2(2).Of these 1420molecules,1079(76.0%)belong to high-symmetry point groups[de?ned as point groups other than C i "1 ,C s(m),C2(2) or C1(1)].There are2299molecules with more than?ve atoms found in structures with no crystallographic symmetry(space group P1).Of these structures,only167(7.2%)belong to high-symmetry point groups(as de?ned above);a very large proportion(84.1%)belong to the point group C(1).Therefore there seems a clear preference for high-symmetry space groups to accommodate molecules of a high symmetry.The space group P1,with no crystallographic symmetry,has a clear preference for molecules with low or no symmetry.It was established in the previous example that metal-containing complexes are found in point groups characterized by opera-tions other than the identity operation more often than organic molecules.In CSDSymmetry,54.3%of the entries with more than?ve atoms contain a metal atom.Of the1420 molecules with more than?ve atoms found within space groups that contain both of the special positions of symmetry C s(m)and C2(2),a large proportion(70.9%)contain a metal atom.The molecules encompassed by the space group P1are predominantly organic;only32.6%contain a metal atom.The lower than expected representation of symmetric metal complexes in P1(and conversely the higher than expected proportion of metal complexes in symmetric space groups)is an indication that distinct relationships between point-group and space-group symmetry exist.These relationships will be expounded on in later publications.

Example4:Which point groups have the greatest proportion of molecules in non-centrosymmetric space groups?As mentioned earlier,crystallization in a non-centrosymmetric

Figure4

Histogram showing the distribution of metal-containing and organic molecules through a subset of space groups.The space groups shown are the six most populated for the entire CSDSymmetry database.Black: metal-containing molecules.Grey:organic molecules.

5The metal-presentˉag(Mpres)is Refcode-speci?c.Thus,if a structure contains four residues and three residues contain a metal atom,all entries in CSDSymmetry for the Refcode have the metal-presentˉag set to one.A subset of CSDSymmetry can be generated for which the number of residues in the structure equals one,and therefore if Mpres=1,a metal is present in that residue.

space group is a requirement for non-linear optical behaviour. The number of molecules,grouped by point group,contained in space groups that do not contain a centre of inversion has been determined and compared with the number of molecules found in each point group for the entire CSDSymmetry database.A user-de?ned equation was formulated to calculate the percentage of molecules of a particular point group that are found in non-centrosymmetric space groups:the ten point groups with the highest percentage of molecules in non-centrosymmetric space groups are given in Table2.A criterion was placed on the query that there had to be more than30 occurrences of the point group in the whole database.No criterion concerning the number of atoms was applied.The molecular point group that contains the highest proportion (43%)of its molecules within non-centrosymmetric space groups is C4(4).Is this an indication that the synthesis of molecules with C4(4)symmetry is a promising route to materials that form in non-centric space groups and that may behave as non-linear optics?

4.Conclusions

Molecular and crystallographic symmetry properties have been extracted from the CSD and collected together in a relational database.This is the most complete collation of observed molecular and crystallographic symmetry properties to date,comprising nearly200000entries.The software package Microsoft Access,which was used to create the CSDSymmetry database,provides a veryˉexible way of creating queries and analyzing the data contained within.The database can be customized by the user with the addition of further?elds or tables.The inclusion of CSD Refcode entries in the database allows the user to(a)match the results of ConQuest searches with information contained within the database and(b)view the results of queries in the molecular visualization package Mercury1.0.It is hoped that the collation and publication of the information contained within this database will provide the scienti?c community with the tool it needs to access and investigate relationships between molecular and crystallographic symmetry properties.

5.Technical details

The entire symmetry database is contained in one Microsoft Access?le,CSDSymmetry.mdb.This?le is approximately 60MB.A self-extracting archive(20MB)has been created, which contains the database and a short help?le.This help?le contains two further example searches with a step-by-step guide as to how to perform them.Microsoft Access has its own help facilities that give information on how to use the software package and detailed information on how to construct queries. The download package will be free for non-commercial research purposes and will be available at the URL http:// https://www.wendangku.net/doc/de7320456.html, in due course.

JAKH thanks the Engineering and Physical Sciences Research Council(UK)for the award of a Senior Research Fellowship.

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Table2

The results of a query constructed in the Microsoft Access database, CSDSymmetry,that determines which point groups contain the highest proportion of their molecules in non-centrosymmetric space groups.

Point group %of molecules found in space

groups that do not contain a

centre of inversion

Number of molecules

in CSDSymmetry

C442.970

C341.11410 C130.7115826 S429.51162 D326.0580

T d24.46220 D223.6977

C223.113474 D5h23.196

D2d21.91420