J. Sep. Sci. 2005, 28, 73–77 www.jss-journal.de i 2005WILEY-VCH Verlag GmbH&Co. KGaA,Weinheim

Original

Pap

er

Xia, Guo,Wang,Wang, Zuo 73

Yan Xia1

YunfengGuo2

HuiWang1

QianWang1

Yumin Zuo1

1Department of Chemistry, NankaiUniversity, Tianjin 300071,People’s Republic of China

2Tianjin Agriculture Bureau,Tianjin 300061, People’sRepublic of China

Quantitative structure-retention relationships ofbenzoylphenylureas on polystyrene-octadecene-encapsulated zirconia stationary phase in reversed-phase high performance liquid chromatography

Quantitative Structure-Retention Relationships of benzoylphenylureas and similarcompounds have been studied on a new type of stationary phase (polystyrene-octa-decene-encapsulated zirconia) in reversed-phase high performance liquid chromato-graphy. Using stepwise regression analysis, the relationships between the structuraldescriptors of the compound and its chromatographic retention were examined. Itwas confirmed that the retention of the dihalogenobenzoylphenylureas is mainlygoverned by the dipole moment (DM), the calculated molar refractivity (CMR), andthe hydrophobicity parameter (C log P) of the compounds. The predicting equationsfor the 18 dihalogenobenzoylphenylureas and similar compounds were also estab-lished; there is a good agreement between the observed k values and the predicted kvalues. In addition, a typical ODS (Kromasil-C18-SiO2) was compared with C18-PS-ZrO2. The results showed that there were some differences between the two station-ary phase for the separation of the compounds investigated, which are certainly relat-ed to surface characteristics of the two different supports.

Key Words: Reversed-phase liquid chromatography; Quantitative structure-retention relation-ships; Octadecene-polystyrene encapsulated zirconia; Benzoylphenyl ureas and similar com-pounds;

Received: October 28, 2003; revised: March 3, 2004; accepted: October 28, 2004

DOI 10.1002/jssc.200301704

1 Introduction

In recent years, a number of chemists have given particu-lar attention to zirconia-based stationary phase for HPLC,since zirconia supports have excellent chemical stabilityand unique surface chemistry [1–4]. In addition, theirretention characteristics were compared with those ofconventional bonded phases [5]. Many methods havebeen used since the 1980’s to evaluate the characteristicsof the stationary phase and predict the retention of thecompound on the stationary phase. Among these, Quanti-tative Structure-Retention Relationships (QSRR) haveproved valuable. The most direct and important applica-tion of QSRR studies is the prediction of the retentionbehavior of substances [6–9]. In QSRR studies, twokinds of input data are needed: one kind is chromato-graphic retention data for a number of studies, serving asdependent variables; the other is accurate solute-related

parameters, which can reflect the structural features ofthe solute being studied, serving as independent vari-ables. QSRR equations can then be obtained using com-putational techniques. In order to find the best relationshipbetween chemical structure and the property of com-pounds, many molecular parameters have been intro-duced and tested in QSRR studies [10], such as non-spe-cific parameters [11–13], geometry-related parameters[14–15], physicochemical parameters [16], topologicalindices [17–20], or combinations of several descrip-tors [21, 22].

Diflubenzuron insecticide and other benzoylphenylureas,effectively used as chitin synthesis inhibitors, have highselectivity and low acute toxicity for mammals [23, 24].Their Structure-Activity Relationships had been studiedby Hajjar et al. [25]. The purpose of this paper was todetermine the retention properties of dihalogenobenzoyl-phenylureas on a new stationary phase, viz. polystyrene-octadecene-encapsulated zirconia (C18-PS-ZrO2), and toestablish the quantitative relationships between the chro-matographic retention and the structure descriptors of thecompounds.We also compared the new phase with a typi-cal ODS (Kromasil-C18-SiO2) to investigate the stationaryphase properties in depth.

Correspondence: Yumin Zuo, Department of Chemistry, NankaiUniversity, Tianjin 300071, People’s Republic of China.Phone: +86 22 23502419. Fax: +86 22 23502458.E-mail: yumin_zuo@nankai.edu.cn.

Abbreviations: RPLC, Reversed-Phase Liquid Chromatogra-phy; QSRR, Quantitative Structure-Retention Relationships; C18-PS-ZrO2, Polystyrene-octadecene-encapsulated zirconia

74 Xia, Guo,Wang, Wang, Zuo

2 Experimental

2.1 Equipment and testing condition

A Varian 5060 Chromatograph, equipped with an UV-100detector, was used to perform all measurements. The zir-conia support was synthesized by a modified polymeriza-

tion-induced colloid aggregation method [26]. C18-PS-ZrO2 was prepared according to a method similar thatdescribed in our earlier work [27]. A C18-PS-ZrO2 column(150 64.6 mm ID) was filled with the aid of a Model6752B-100 high pressure pump, made by Beijing Analyti-cal Instrument Technical Company, Beijing, China. A Kro-masil-C18-SiO2 column (15064.6 mm ID) was comparedwith the C18-PS-ZrO2 column we prepared. Test condi-tions: the flow rate was 1 mL/min, the mobile phase wasmade by mixing methanol with water in different propor-tions, the determination temperature was 258C unlessotherwise stated. The UV detector was set at 254 nm. Thedead time was determined by injecting acetone. An SGIIndy Workstation (USA) with Sybyl 6.22 Software (TriposCompany) was used for data collection, and a PCPentiumComputer was used for data processing.

2.2 Compounds tested

Eighteen dihalogenobenzoylphenylureas were used astest compounds; their structures are given in Table 1.These compounds were synthesized by reaction of corre-sponding benzoyl isocyanates with related aniline deriv-atives and verified by elemental analysis. Prior to use, thetest compounds were dissolved in chromatographic grademethanol; their concentrations were 0.1–1.0 mg/mL.

J. Sep. Sci. 2005, 28, 73–77 www.jss-journal.de i 2005WILEY-VCH Verlag GmbH&Co. KGaA,Weinheim

Table 1. Structure of the tested compounds.

Table 2. Structural descriptors of the test compounds.

No. FHF[kcal/mol]

ECCR[eV]

HOMO[eV]

DM[debye]

CMR C log P

1 27.75 2.3856104 –9.838 10.02 8.530 3.630

2 234.3 344.2 –10.89 3.010 8.530 3.630

3 17.00 2.3476104 –9.861 6.829 8.530 3.630

4 –28.01 2.3806104 –9.532 7.334 9.000 3.880

5 30.38 2.9586104 –10.013 6.016 9.140 3.510

6 234.3 344.2 –9.669 7.057 8.410 4.250

7 –220.5 3.2516104 –9.723 6.190 9.060 5.210

8 21.28 2.0076104 –9.652 6.595 8.700 4.400

9 6.976 2.0116104 –9.569 6.532 8.410 4.250

10 –102.3 2.4196104 –9.440 7.843 8.050 3.580

11 –67.29 2.0466104 –9.567 7.404 7.460 3.950

12 –53.09 2.0416104 –9.550 6.939 7.740 4.100

13 –44.91 2.9916104 –10.62 6.432 8.190 3.210

14 –57.63 2.3786104 –10.05 7.137 7.580 3.330

15 –58.43 2.3766104 –10.20 2.399 7.580 3.330

16 –51.99 2.4396104 –9.909 10.96 7.580 3.330

17 –301.4 3.7256104 –9.810 5.634 9.090 6.070

18 –67.37 2.0456104 –9.464 6.878 7.460 3.950

Descriptor data of TE, EE, LUMO were deleted to reduce the length.

Quantitative structure-retention relationships of benzoylphenylureas 75

2.3 Structural descriptors

The structural descriptors of the test compounds areshown in Table 2, where FHF is the final heat of forma-tion, TE is the total energy, EE is the electronic energy,ECCR is the energy of core-core repulsion, HOMO is theenergy of the highest occupied molecular orbital, LUMO isthe energy of the lowest unoccupied molecular orbital,DMis the dipole moment, CMR is the calculated molar refrac-tivity, C log P is the hydrophobicity parameter; they wereall calculated with the aid of the SGI IndyWorkstation.

3 Results and discussionEach log k value for the compounds tested on C18-PS-ZrO2 stationary phase and Kromasil-C18-SiO2 is listed inTable 3, where the mobile phase is a mixture of methanol/water in different proportions (80 :20, 85:15, 90:10,95 :5). The QRSS equations found for the two stationaryphases using stepwise regression analysis are shown inTable 4.

3.1 Comparison of the fitting coefficients betweenC18-PS-ZrO2 stationary phase and Kromasil-C18-SiO2

From Table 4, we find that log k values have a better cor-relation with DM, CMR, and C log P than with other

parameters when the stationary phase is C18-PS-ZrO2. Itis noted that CMR reflect that the refractive index of acompound, while DM and C log P reflect the polarity andhydrophobicity of compounds, respectively. From thecoefficients of the three factors in the equations, we cansee that CMR has higher coefficients than DM andC log P. This indicates that the effect ofCMR on the reten-tion of these compounds is more important than that ofDM and C log P. However, when the stationary phase isKromasil-C18-SiO2, there are some differences: at lowermethanol content (80%, 85%) of mobile phase, log kvalues correlate with C log P, DM, and FHF, while at highmethanol content in the mobile phase, the log k value cor-relates not only with C log P, DM, and CMR, but also theelectronic properties HOMO and ECCR. The fact that thecoefficients are not as good as for the other stationaryphase indicated that the 9 structural descriptors selectedin this paper may not be the dominant factors for retentionof these benzoylphenylureas on ODS stationary phase.

In order to explain the differences between the conven-tional silica-based bonded phase and the C18-PS-ZrO2

phase, we compare the QSRR coefficients found for thetwo phases: the coefficients of the two phases are verydifferent, especially at high methanol content; these differ-ences may be due to the acidity and hydrophobicity of the

J. Sep. Sci. 2005, 28, 73–77 www.jss-journal.de i 2005WILEY-VCH Verlag GmbH&Co. KGaA,Weinheim

Table 3. log k values of each compound.

Compdstested

C18-PS-ZrO2 Kromasil-C18-SiO2

95%CH3OH

90%CH3OH

85%CH3OH

80%CH3OH

95%CH3OH

90%CH3OH

85%CH3OH

80%CH3OH

1 –0.004100 0.2827 0.5479 0.8217 –0.6714 –0.5157 –0.2566 –0.02220

2 0.3101 0.6572 1.214 – –0.4683 –0.2960 –0.01150 0.2546

3 0.2391 0.4711 0.8596 1.274 –0.5241 –0.3378 –0.06430 0.1918

4 0.3775 0.5546 0.8099 1.253 –0.4189 –0.2258 0.04880 0.3162

5 0.3549 0.6489 1.080 1.341 – 0.5950 –0.3680 –0.0532 0.2150

6 0.2531 0.4567 0.8621 1.205 –0.2701 –0.04410 0.2488 0.5160

7 0.3697 0.6993 1.0899 – –0.3406 –0.00506 0.4441 0.8051

8 0.2712 0.5530 0.9738 1.420 –0.2507 –0.01810 0.3122 0.5712

9 0.2102 0.4479 0.8899 1.322 –0.3802 –0.04410 0.2396 0.4943

10 0.06740 0.3040 0.4897 0.9707 –0.5745 –0.3602 –0.2867 0.1401

11 –0.04020 0.2152 0.5801 1.001 –0.4236 –0.1829 0.0888 0.3309

12 –0.005000 0.2089 0.7101 1.332 –0.3500 –0.1079 0.1557 0.3265

13 0.1710 0.3633 0.7697 1.167 –0.7746 –0.6973 –0.5433 0.05370

14 –0.003000 0.2313 0.5699 0.915 –0.7153 –0.4942 –0.2451 0.04680

15 0.2093 0.5126 0.9601 1.451 –1.346 –0.02410 –1.652 –1.367

16 –0.1869 0.005900 0.2493 0.4926 –0.8311 –0.6424 –0.4127 –0.1700

17 0.3499 0.7629 1.246 – –0.4580 -0.1079 0.2640 0.6195

18 –0.08100 0.2469 0.5998 1.058 –0.4143 –0.1803 –0.06240 0.3207

76 Xia, Guo,Wang, Wang, Zuo

stationary phases. Specifically the C18-PS-ZrO2 phaseacts as a weaker acid than does the C18-SiO2 phase. Thismight be due to differential acidities of residual surfacesilanol groups in comparison to surface hydroxyl groupson the zirconia. The surface of the C18-PS-ZrO2 phase isencapsulated by a polymer layer which increases thehydrophobicity, so it is more hydrophobic than the C18-SiO2 phase, towards the compounds separated. Theretention time on C18-PS-ZrO2 is somewhat longer thanthat on C18-SiO2, probably due to this difference in hydro-phobicity.

3.2 Coefficient analysis of QSRR equations on thestationary phase C18-PS-ZrO2

For a given mobile phase-stationary phase system, posi-tive or negative signs of the coefficients of the structuraldescriptors of the QSRR equation as well as the absolutevalue of the coefficient reflect the interaction of the testcompounds and the mobile phase as well as that betweenthe test compounds and the stationary phase. Theseparameters can also influence the retention of the testcompound on the stationary phase. We can draw the fol-lowing conclusions from Table 4.

3.2.1 Dipolar moment

The value of the coefficient of dipolarity is negative, whichshows that the retention of the solute decreases withincreasing dipolarity. For a given mobile-stationary phasesystem, this coefficient reflects the difference of the inter-action between the solute and the mobile phase orbetween the solute and the stationary phase. The dipolar

moment of water is 1.17, that of methanol is 0.61, that ofbenzene is 0.52, and that of cyclohexane is 0, so for themethanol/water system, the dipolar interaction betweenthe solute and the mobile phase is significant, but for theC18-PS-ZrO2 stationary phase we used, the differencebetween the solute and the mobile phase or the stationaryphase is not very pronounced since the high polarity ben-zoyl group can adsorb a large amount of methanol in themobile phase. In a mobile phase of different methanol/water ratio, an increase in the quantity of adsorbedmetha-nol in the stationary phase will lead to a decrease in thedifference of the dipolar interaction between the mobilephase and the stationary phase and the solute, and theabsolute value of the coefficient will decrease.

3.2.2 Calculatedmolar refractivity

CMR is the refractive index of a material. The lower therefractive index of a substance, the weaker are the disper-sive interactions. The refractive index of water is 1.333,that of methanol is 1.327, that of benzene is 1.501, andthat of octadecane is 1.44. The aqueous mobile phasesare much less dispersive than organic stationary phases.Because the highly polarizable phenyl groups of the aro-matic phase can adsorb a great deal of the organic com-ponents (methanol), the difference is not very pro-nounced; the values of the coefficient remain close toeach other in mobile phases of different methanol content.

3.2.3 Hydrophobicity parameter

In RPLC, the hydrophobic effect can represent the parti-tion of the solute between the mobile phase and the sta-

J. Sep. Sci. 2005, 28, 73–77 www.jss-journal.de i 2005WILEY-VCH Verlag GmbH&Co. KGaA,Weinheim

Table 4. Quantitative relationship between retention and structure of compounds.

Composition of mobile phasemethanol %

QSRR equations

80% C18-PS-ZrO2 log k = –1.386+0.1009C log P(0.4551)* – 0.1073DM(0.0000)* + 0.3253CMR(0.0252)*N = 15,R = 0.981, F = 47.55,SD = 0.06195,P a 0.00001

Kromasil-C18-SiO2 log k = –2.553+0.5528C log P(0.0007)*+0.09014DM(0.0451)*+0.0015FHF(0.0624)*N = 18,R = 0.972, F = 26.53,SD = 0.0614,P a 0.0005

85% C18-PS-ZrO2 log k = –0.7125+0.07474C log P(0.0030)*–0.09142DM(0.0000)*+0.2219CMR(0.0000)*N = 18,R = 0.984, F = 1189.1,SD = 0.01131,P a 0.00001

Kromasil-C18-SiO2 log k = –3.052+0.5891C log P(0.0002)*+0.0996DM(0.0212)*+0.0018FHF(0.0176)*N = 18,R = 0.971, F = 14.856,SD = 0.3030,P a 0.0015

90% C18-PS-ZrO2 log k = –1.256+0.0406C log P(0.0029)*–0.0566DM(0.0000)*+0.2291CMR(0.0000)*N = 18,R = 0.992, F = 277.5,SD = 0.02901,P a 0.00001

Kromasil-C18-SiO2 log k = 4.061+0.1485C log P(0.0011)*–0.08267DM(0.0000)*+0.4004HOMO(0.0000)*N = 18,R = 0.977, F = 34.75,SD = 0.0655,P a 0.00000

95% C18-PS-ZrO2 log k = –0.146+0.2332CMR(0.0000)*–0.0430DM(0.0000)*N = 18,R = 0.975, F = 73.92,SD = 0.04732,P a 0.00001

Kromasil-C18-SiO2 log k = –1.928+0.4577CMR(0.0031)*–0.0002000ECCR(0.0254)*N = 18,R = 0.980, F = 21.566,SD = 0.0703,P a 0.0001

N : number of the compounds tested, R : regression coefficient, F [28]: a statistic for assessing the overall significance, SD : stan-dard deviation, P : significance level, *significant level for individual term.

Quantitative structure-retention relationships of benzoylphenylureas 77

tionary phase. It also relates to the standard free energywhen the solute moves from water to organic solvent.Generally speaking, the hydrophobic effect relates to theincrease in free energy and heat capacity that areobserved when a non-polar moiety (atom, molecule, orfragment of a molecule) is transferred from a non-polarenvironment to a polar environment such as water andmethanol. Since the transfer of solute to water is attendedby a large negative entropy change and results in a largerpositive total free energy change, this process is unfavor-able, and is characterized by the term hydrophobicity. Inthe QSRR equation we established that the coefficient ofC log P decreases with increasing methanol content.When the content of methanol increases to 95%, C log Pbecomes unimportant and can be neglected, the CMR isstill the most retention-influencing parameter for thesecompounds.

4 Concluding remarksIn conclusion, on a C18-PS-ZrO2 stationary phase, therelationship between the retention factor (k) and the threestructure parameters (DM, CMR, and C log P) of dihalo-genobenzoylphenylureas has been established. Goodagreement was obtained between predicted and experi-mental results. It is thus possible to predict the retention ofcompounds having similar structures on the C18-PS-ZrO2

stationary phase, which is of considerable interest inmolecular design investigations and evaluation of station-ary phases.

AcknowledgmentThis work was supported by the National Natural ScienceFoundation of China, project number 20275018.

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