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Quantitative Evaluation of Dissociation Mechanisms in Phenolphthalein and the Related Compounds

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Quantitative Evaluation of Dissociation Mechanisms in Phenolphthalein and the Related Compounds J. Comput. Chem. Jpn., Vol. 15, No. 1, pp. 13–21 (2016) ©2016 Society of Computer Chemistry, Japan Technical Paper Quantitative Evaluation of Dissociation Mechanisms in Phenolphthalein and the Related Compounds Toshihiko HANAI Health Research Foundation, Research Institute for Production Development 4F, 15 Shimogamo-morimoto-cho, Sakyo-ku, Kyoto 606-0805, Japan e-mail: [email protected] (Received: October 11, 2015; Accepted for publication: April 14, 2016; Online publication: May 6, 2016) Computational chemistry programs were evaluated as aids to teaching qualitative analytical chemistry. Compu- tational chemical calculations can predict absorption spectra, thus enabling the modeling of indicator dissociation mechanisms by different computational chemical programs using a personal computer. An updated MNDO program among 51 programs was found to be the best predictor to explain the dissociation mechanisms of isobenzofuranones and sulfonephthaleins. Unknown dissociation constants were predicted from atomic partial charges instead of Ham- mett's constants. Keywords: Quantitative analysis of dissociation mechanisms, Isobenzofuranone, Sulfonephthalein, Computational chemistry 1 Introduction the absorption wavelength and electron density changes were not well described. The dissociation mechanisms, maximum How to quantitatively teach qualitative analytical chemistry is wavelengths, and electron density maps of isobenzofuranones a very important subject for analytical chemists. Previously, a and sulfonephthaleins were, therefore, evaluated by in silico method to teach molecular interaction mechanisms in chroma- analysis despite the anticipated poor precision. The experimen- tography was quantitatively achieved using molecular mechan- tally measured dissociation of phenolphthalein is described ics and MOPAC programs [1]. Furthermore, the reaction mech- using four dissociation structures, where the ionization of two anisms of highly sensitive detections were also quantitatively phenolic hydroxyl groups converts the neutral molecular form described [2,3]. Further study was carried out for simple detec- into the red quinoid form. Further dissociation from the quinoid tion, indicator's color changes by using updated computational structure to the alcoholic form eliminates the color. The disso- chemical programs. ciation mechanisms can also be described using three structures Color indicators have been the backbone of simple pH tests without the need for a transition structure [8]. and titration analyses. The spectrophotometric determination There are many color indicators having chemical structures of hydrogen ion concentrations by using color indicators was similar to phenolphthalein; these indicators should have similar described [4]. The precision of indicator dissociation constants dissociation mechanisms. The differences in their dissociation was evaluated and the dissociation mechanisms (pKa) were de- constants depend on the inductive effects of the substituents. scribed in detail. The pKa values were found to vary according The associated four dissociation structures were constructed to salt, temperature, and the laboratories where the work was and their spectra and HOMO and LUMO electron density maps conducted [5,6]. The effects of salts and proteins on the spectra were calculated. The unreported dissociation constants were of some dyes and indicators were studied [7]. The dissociation predicted from atomic partial charge calculated using an empir- processes were described in detail by Kolthoff [5]. However, ical program PM6. There still remains the limitation that com- DOI: 10.2477/jccj.2015-0055 13 putational chemistry programs can estimate the spectra only in phenyl)-7,7-dioxo-8-oxa-7λ6-thiabicyclo[4.3.0]nona- the absence of solvents. 1,3,5-trien-9-yl]-5-methyl-2-propane-2-yl-phenol 2 Experimental 3 Results and Discussion The computers used were PowerMac G3 and Dell Optiplex The basic structures of isobenzofuranones and the dissoci- running on ChemIntosh® from SoftShell (SanDiego, CA) and ated phenolphthalein molecules drawn using Chemintosh® are CAChe® and SciGress® programs from Fujitsu (Tokyo, Japan). shown in Figures 1 and 2, respectively. There are many com- The spectra were measured in aqueous solutions using a Shi- putational chemistry programs available to create electronic madzu UV1200 (Tokyo, Japan). Indicators used are listed be- spectra. The programs used are summarized in Table 2. The low. The SciGress program provides different programs to cal- spectra of phenolphthalein structure 3, shown in Figure 3, were culate electronic spectra, including MNDO, AM1, PM3, PM5, calculated to compare MM2 and MM3 using programs 3, 4, PM6, RM1, PDDG/MNDO, PDDG/PM3, DFT, ZINDO/S, and 25 (a, b). Structure 3 was initially optimized using MM2 INDO/S, CNDO/S, CNDO/S2, CNDO/S3, CNDO/2, CI, RPA, for programs 3 and 25 (a), and MM3 for programs 4 and 25 (b). ZINDO, and MO-S. The dissociation spectra of phenolphtha- Programs 3 and 4 are ZINDO and program 25 is MO-S MNDO. lein structures were calculated using these programs and used The basic geometries can be optimized using molecular me- to evaluate the accuracy of the programs. The properties of the chanics, either MM2 or MM3. MM3 was found to show weak indicators are summarized in Table 1. The abbreviations of absorption spectra, and did not provide definite information computational chemical programs are summarized later. about the maximum absorption wavelengths. Structures opti- mized using MM3 exhibited weak molar absorptivities and red 2.1 List of Indicators shift. The intensity was about half of that of calculated using o-Cresolphthalein:3,3-bis(4-hydroxy-3-methylphenyl)-iso- MM2 as shown in Figure 3, and semi-empirical combinations benzofuran-1(3H)-one did not work in many cases, with computer error messages be- α-Naphtholphthalein:3,3-bis(4-hydroxynaphthalene-1-yl)-2- ing generated. Therefore, MM2 was chosen as the initial pro- benzofuran-1-one gram, and the spectra of dissociated phenolphthaleins were cal- Phenolphthalein:3,3-bis(4-hydroxyphenyl)isobenzofuran- culated using SciGress programs. The results are summarized 1(3H)-one Thymolphthalein:3,3-bis(4-hydroxy-2-methyl-5-propane- in Table 3, where some absorption wavelengths are given fol- 2-ylphenyl)-2-benzofuran-1-one lowing their absorption strength. The three dimensional struc- Bromocresolgreen;2,6-Dibromo-4-[7-(3,5-dibromo- tures of phenolphthalein optimized using the MM2 program are 4-hydroxy-2-methyl-phenyl)-9,9-dioxo-8-oxa-9λ6- shown in Figure 4 with HOMO and LUMO electron density thiabicyclo[4.3.0]nona-1,3,5-triene-7-yl]-3-methylphenol maps optimized using semi-empirical programs (PM6). Bromocresolpurple:4,4'-(1,1-dioxido-3H-2,1-benzoxathi- The estimated wavelength of structure 3 representing the red ole-3,3-diyl)bis(2-bromo-6-methylphenol) color varied from 310 to 813 nm. Absorption spectra are gener- Bromophenolblue:4,4'-(1,1-dioxido-3H-2,1-benzoxathi- ole-3,3-diyl)bis(2,6-dibromophenol) ally shifted to lower wavelength in polar solution. Therefore, Bromothymolblue:4,4'-(1,1-dioxido-3H-2,1-benzoxathi- the calculated wavelengths should be higher than the measured ole-3,3-diyl)bis(2-bromo-6-isopropyl-3-methylphenol) wavelength of 550 nm. Programs 2, 3, 14–16, 18–21, 25, 27, Chlorophenolred:2-chloro-4-[3-(3-chloro-4-hydroxyphenyl)- 28, 32–35, 37, 42, 49, and 50 were acceptable candidates for 1,1-dioxobenzo(c)oxathiol-3-yl]phenol further study. Therefore, the spectra of structures 1 and 4 were Cresolred:4,4'-(1,1-dioxido-3H-2,1-benzoxathiole-3,3-diyl) calculated using these programs. bis(2-methylphenol) The evolution of computational chemistry programs should Cresolpurple:4,4'-(1,1-dioxido-3H-2,1-benzoxathiol-3-yl- dine)bis(3-methylphenol) improve the precision of predicted spectra. However, these re- Phenolred:4,4'-(3H-2,1-benzoxathiole-3-ylidene)bisphenol sults indicate that no single program accurately predicts spectra Thymolblue:4-[9-(4-hydroxy-2-methyl-5-propane-2-yl- for all types of compounds. By comparison of the configuration 14 J. Comput. Chem. Jpn., Vol. 15, No. 1 (2016) Table 1. Properties of indicators interaction (CI) and analogous random-phase approximation structure 3. The combination of RPA and a semi-empirical ge- (RPA) using MM2 geometry, MO-S, RPA, MM2, and AM1, ometry such as AM1, PM3, PM5, or RM1 gave higher wave- or PM3 or PM5 combinations predicted greater red shifts lengths than that of a CI and semi-empirical combination, but than MO-S, CI, MM2, and AM1, or PM3 or PM5 combina- the calculated wavelength were still relatively short. PDDD tions. CNDO/2, CNDO/S2, CNDO/S3, and RPA combinations and MNDO combinations worked fine with CI; however, a were not suitable for phenolphthalein structure 3, but CNDO/2, combination of PDDG/MNDO and RPA showed increased red CNDO/S2, CNDO/S3, and CI combinations estimated the red shifts. In general, CI was better than RPA that gave weak visible shift wavelength quite well. CNDO/S and CI or RPA combina- wavelengths, as well as than RM1. As described in reference 8, tions worked well, but INDO/S and CI or RPA combinations CI was much more accurate than RPA and even predicted the gave overly strong red shift. However, ZINDO gave a relatively wavelength difference. The long calculation times for CI have better wavelength prediction. A Zerner modification seemed to been reduced by the development of fast personal computers. improve the original INDO performance for phenolphthalein These results differed from those of six carotenoids [17]. In DOI: 10.2477/jccj.2015-0055 15 Figure 1. Chemical structures of isobenzofurans. Figure 2. Dissociation scheme of phenolphthalein. calculations of the electronic wavelength spectra of carotenoids in conjugated molecules, INDO/S, provided the best agree- semi-empirical geometries (AM1, PM3, PM5, PM6, or RM1) ment with the experiment. AM1 and PM3 values were very demonstrated lower absorption wavelengths than with either similar with a small shift of PM3 energies toward the blue light. MM2 or MM3 geometries. MO-S with a semi-empirical pro- Compared with INDO/S, AM1 and PM3 results were shifted gram with MM2 or MM3 geometries showed higher absorption either to the blue or red, depending on the molecules chosen.
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