Structure and Property Investigations of TDO in Aqueous Phase by

Article pubs.acs.org/JPCA

Structure and Property Investigations of TDO in Aqueous Phase by Density Functional Theory, UV Absorption, and Raman Spectroscopy Jianzhong Shao,‡,§ Xiaoyun Liu,‡,§ Pin Chen,† Qiuxia Wu,† Xuming Zheng,†,‡,§ and Kemei Pei*,†,‡,§ †

Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education of China, Zhejiang Sci-Tech University, Hangzhou 310018, China § Engineering Research Center for Eco-Dyeing and Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China ‡

S Supporting Information *

ABSTRACT: Density functional theory, UV absorption, and Raman spectroscopy are used to investigate the structure and properties of TDO in aqueous solution. The equilibrium structures, UV absorption spectra, interaction energies, and Raman spectroscopy data of TDO, AIMSA, and 12 TDO or AIMSA clusters are calculated. Raman spectroscopy experiments are carried out by 488 and 208 nm laser excitation. The Raman spectra of TDO in solid and aqueous phases have been compared, and the most possible structure for TDO in aqueous phase was deduced from analysis of the DFT calculations for the examined models, the experimental UV absorption spectrum, and Raman spectra of TDO. The interaction energy results show that TDO’s solubility in water is originated from the TDO−water cyclic oligomer. The calculated UV absorption and Raman spectra of the I2·2H2O-cyc cluster model agree with the experimental results of TDO in aqueous solution very well.

1. INTRODUCTION Thiourea oxides, especially thiourea dioxide (abbreviated as TDO), have received much attention for their wide applications in chemistry and chemical industry.1,2 Numerous commercial and industrial applications of TDO are correlated with their strong reducing property, excellent stability, and safety.3 Thus, TDO has been frequently used to reduce some organosulfur, organoselenium, organotellurium, and organfluorine compounds and some nitrogen-containing compounds or metals, or it has been used as a component of an effective initiation system for polymerization reactions.2,4 In the textile industry, TDO is widely used as an excellent bleaching agent because it is green to the environment, and an attempt to use TDO as a discharging agent in textile discharge printing has been recently reported.5 It is well known that the solid TDO exists in (NH2)2CSO2 form by XRD technique, and the TDO molecules are linked by a system of hydrogen bonds.6 Makarov et al. considered tautomerization of TDO in aqueous solution followed by rearrangement of its linear TDO−water oligomer at a low precise semiempiritical AM1 theoretical level because the changes in the 1H NMR spectra of TDO with time are not closely related to the formation rate of aminoimino methanesulfinic acid (AIMSA).7,8 TDO is very stable in acidic aqueous solvent, but AIMSA is unstable in aqueous solvent.4,7 All of these facts show that the exact structure of TDO in © 2014 American Chemical Society

aqueous phase is not clear. It is well known that Raman spectroscopic technique is very suitable to investigate the aqueous system because Raman scattering signals of water are very weak. Combination of high-accuracy quantum chemistry calculations, UV absorption, and Raman spectroscopy is very powerful to investigate the molecular structure from the previous reports. In this work, Raman spectra of TDO in aqueous and solid phases were obtained with 488 nm laser excitation. For TDO’s weak solubility and low molar extinction coefficient above the 220 nm optical region, here we also selected 208 nm as the excitation laser for TDO in aqueous solution to get resonance Raman spectroscopy. Additionally, the structural changes between the aqueous and solid phases based on Raman spectra data are discussed in detail.

2. EXPERIMENTS AND CALCULATIONS The 208 nm Raman spectrum was obtained by the resonance Raman experimental apparatus, which has been described elsewhere.9,10 208 nm excitation laser was generated by the harmonics of a nanosecond Nd:YAG laser and their hydrogen Raman-shifted laser lines. 488 nm laser was produced by CVI Received: December 27, 2013 Revised: April 7, 2014 Published: April 8, 2014 3168

dx.doi.org/10.1021/jp502375r | J. Phys. Chem. A 2014, 118, 3168−3174

The Journal of Physical Chemistry A

Article

Figure 1. Simulation models for TDO in aqueous solution at B3LYP/cc-PVDZ theoretical level

Melles Griot argon ion laser (543-AP-A01), and Raman backscattering signals were collected. It has been proved that the DFT method supplies excellent results in many hydrogen-bonding systems.11,12 In this work, the Becke proposed hybrid (B3) together with the LYP correlation functional and the basis set cc-PVDZ were chosen.13,14 The initial structures of 14 simulation models were constructed by Chemoffice software. Once structural optimization was completed, the harmonic frequencies were examined to confirm whether the point was a true minimum. The interaction energies were obtained by subtracting the energies of the fully optimized monomers from the total energy of the cluster, and basis set superposition errors (BSSEs) were estimated by the standard counterpoise (CP) method. 15 All calculations were carried out with the Gaussian 09 package at 298.15 K. 16

Table 1. Point Group, C−S Bond Length, and Dipole Moment of All Simulation Models

3. RESULTS AND DISCUSSION 3.1. Simulation Models. In this work, TDO(I), AIMSA(II), and 12 TDO or AIMSA clusters are simulated to get insight into the structure and properties of TDO solubilized in water, as clearly shown in Figure 1. The equilibrium structures of all models are optimized at the B3LYP/cc-PVDZ theoretical level, and the vibrational frequencies are calculated at the same theoretical level to check the equilibrium structure and to get

simulation models

point group

C−S bond length

dipole moment

I II H2O I·H2O II·H2O I·2H2O II·2H2O I2-cyc I2-line I2·2H2O-cyc I2·2H2O-line I3-cyc I3.3H2O-cyc I4-cyc I5-cyc

C1 C1 C2v Cs C1 Cs C1 C2h Ci C2 Ci C3h C1 C4h C5h

2.19 1.91

7.53 1.38 1.94 6.38 1.67 5.75 6.23 0.00 0.00 2.35 0.00 0.00 0.90 0.00 0.00

2.03 1.90 2.04 1.91 1.97 1.97 1.94 1.95 1.92 1.92 1.91 1.91

the calculated Raman spectrum. To the initial geometries, various hydrogen-bond interactions (the lone pairs on water, oxygen, and nitrogen atoms of TDO and AIMSA for accepting hydrogen bonds) are allowed, and Figure 1 shows the optimized structures of all simulation models. Table 1 lists 3169



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Table 2. B3LYP/cc-PVDZ-Calculated and Experimental Maximum Absorption Wavelength (nm) above 220 nm, Oscillator Strength f, and Molar Absorption Coefficient (L·mol−1·cm−1) for 14 Simulation Modelsa molar absorption coefficient transition state I S0→S1 S0→S3 S0→S4 II S0→S1 S0→S2 I·H2O S0→S2 S0→S3 II·H2O S0→S1 I·2H2O S0→S1 S0→S2 S0→S3 II·2H2O S0→S1 S0→S2 I2-cyc S0→S2 S0→S4 S0→S8 I2-line S0→S4 S0→S6 S0→S10 I2·2H2O-cyc S0→S4 I2·2H2O-line S0→S1 S0→S4 S0→S6 S0→S10 I3-cyc S0→S1 S0→S2 S0→S5 S0→S6 I3·3H2O-cyc S0→S1 S0→S2 S0→S8 I4-cyc S0→S1 S0→S2 S0→S3 S0→S4 S0→S5 S0→S9 S0→S10 I5-cyc S0→S4 S0→S5 S0→S9 S0→S10 a

maximum absorption wavelength

f

calc.

expt.

calc.

expt.

0.0549 0.0020 0.0227

2200

550 (in water)

285.10 251.88 221.75

269 (in water)

0.0152 0.0130

600

271.98 224.95

0.0586 0.0018

2400

256.07 230.35

0.0177

250

254.13

0.0203 0.0845 0.0035

3500

299.36 246.88 232.18

0.0070 0.0048 0.0107 0.0676 0.0423

300

264.57 232.26 302.65 255.08 224.40

2800

0.0089 0.0891 0.0116

1600

254.11 237.48 221.41

0.0228

900

253.20

0.0050 0.0120 0.1048 0.0355 0.0057 0.0056 0.0077 0.0076 0.0130 0.0113 0.0052 0.0000 0.0228 0.0227 0.0430 0.0430 0.0091 0.0091 0.0430 0.0430 0.0091 0.00914

290.83 258.50 234.52 220.43

2000

289.79 289.67 253.01 252.83

1100

284.18 280.89 239.20

1100

287.34 285.95 285.80 286.11 286.10 258.55 258.54

2000 4000

286.11 286.10 258.55 258.5

3700

Only oscillator strength f values above 0.0050 are list in Table 2. 3170



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absorption spectrum for TDO in the aqueous phase, we can speculate that II, II·H2O, II·2H2O, and I2·2H2O-cyc models are the possible structures for TDO in aqueous solution. 3.3. Energy Analysis. Table 3 lists the total energies and the interaction energies of all simulation models, which are calculated at the B3LYP/cc-PVDZ theory level. ΔE NCP (without BSSE correction) and ΔECP (with BSSE correction) refer to the interaction energies. The total energies of I (TDO) and II (AIMSA) in Table 3 indicate that in the gas phase AIMSA is more stable than TDO. To the cluster models, a meaningful conclusion can be drawn that more strong interactions are found for TDO interacting with itself or water molecule than AIMSA interacting with itself or water molecule. For example, the interaction energies (ΔECP) of I· H 2 O and I·2H 2 O are −16.12 and −31.82 kcal/mol, respectively, but only −11.85 and −18.23 kcal/mol are found for II·H2O and II·2H2O. An interesting phenomenon is found in this work for TDO polymers in cyclic structure, such as I2cyc, I3-cyc, I4-cyc, and I5-cyc, and these polymers show very strong interactions because TDO can interact with itself by hydrogen bond, and these calculation results can explain why TDO in solid state is in (NH2)2CSO2 form and very stable. Table 3 clearly presents that there are strong interactions between TDO and water molecule, but more strong interaction was found when TDO interacts with itself. As we know, the solubility of TDO in water is as low as 26.7 g/L (0.25 mol/L) at 293.15 K. The reason that the low solubility of TDO in water may be because the hydrogen-bond interaction of TDO powder is too strong to break by water molecule easily. I2· 2H2O-cyc and I3·3H2O-cyc have very strong interaction energies. The interaction energy of I2·2H2O-cyc (ΔECP: −75.79 kcal/mol) is much stronger than that of I2·2H2O-line (ΔECP: −57.60 kcal/mol). These calculation results show that when TDO solubilizes in water the formation of the cyclic structures constructed by water and TDO is more possible than the formation of linear structures. Makarov et al. considered that TDO’s solubility in water is originated from the TDO oligomer, and these TDO low polymers have linear polymer structures. 6 Our energetic results show that the oligomers constructed by TDO and water in cycle are more possible than the linear oligomers. 3.4. Raman Spectra. Figure 3 presents the calculated, 488 nm, and 208 nm Raman spectra of TDO in two phases, and asterisks label parts of the spectra where solvent subtraction artifacts or laser line are present. Raman spectra of TDO in solid and aqueous phases are obtained by 488 nm laser excitation, and 208 nm deep UV resonance Raman experiment for TDO in aqueous phases was also carried to help analyze the vibrational modes. Table 4 lists the experimental Raman and B3LYP/cc-PVDZ calculated vibrational frequencies for TDO in the two different environments of interest. The notations and assignments of the vibrations are based on the visualization by Gaussview5 software. According to the UV absorption analysis, II, II·H2O, II· 2H2O, and I2·2H2O-cyc models are possible structures for TDO in aqueous solution. Therefore, only the calculated vibrational frequencies of II, II·H2O, II·2H2O, and I2·2H2O-cyc models are list in Table 4. The calculated results show that there is no 1425 cm−1 vibrational frequency for II, II·H2O, and II·2H2O models, but I2·2H2O-cyc model has 1425 cm−1 vibrational mode. Additionally, the calculated Raman spectrum of I2·2H2O-cyc model agrees with the 208 and 488 nm Raman spectra very well, as Figure 3 shows. The above energy result

Figure 2. UV absorption spectrum of TDO in aqueous solution

Table 3. Total Energies and Interaction Energies of All Simulation Models combination energy (kcal/mol) simulation model

Etotal (hartree)

ΔENCP

ΔECP

I II H2O I·H2O II·H2O I·2H2O II·2H2O I2-cyc I2-line I2.2H2O-cyc I2.2H2O-line I3-cyc I3.3H2O-cyc I4-cyc I5-cyc

−698.6105794 −698.6208920 −76.42062630 −775.0660965 −775.0693675 −851.5115569 −851.5016183 −1397.2926595 −1397.2760739 −1550.1958262 −1550.1654727 −2095.9901819 −2325.3162318 −2794.6607365 −3493.3243088

−21.89 −17.47 −37.47 −24.77 −44.86 −34.45 −83.71 −64.67 −99.42 −139.69 −137.05 −170.31

−16.12 −11.85 −31.82 −18.23 −38.99 −27.66 −75.79 −57.60 −92.18 −131.23 −128.91 −160.22

the point group, C−S bond length, and dipole moment of all simulation models. The results in Table 1 clearly show that the C−S bond length obviously becomes short when TDO molecule interacts with water molecule or itself. 3.2. UV Absorption Spectra. Table 2 lists the B3LYP/ccPVDZ calculated and experimental maximum absorption wavelength above 220 nm, oscillator strength f, and molar absorption coefficients for 14 TD simulation models. As we know, TDO has a relative strong absorption band near 269nm with molar absorption coefficient of ∼550 L·mol−1·cm−1 in aqueous solution, as Figure 2 shows. Table 1 clearly shows that the maximum absorption wavelengths above 220 nm and the molar absorption coefficients of II, II·H2O, II·2H2O, and I2· 2H2O-cyc models generally agree with the experimental results. The maximum absorption wavelengths above 220 nm of I, I· H2O, I·2H2O, I2-cyc, I2-line, I2·2H2O-line, I3·cyc, and I3·3H2Ocyc agree with the experimental results, but their molar absorption coefficients deviate from the experimental values significantly. The maximum absorption wavelengths and the molar absorption coefficients of I4-cyc and I5-cyc obviously disagree with the UV absorption spectrum of TDO in water. From the theoretical and experimental results about UV 3171



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Figure 3. Calculated, 488 nm, and 208 nm Raman spectra of TDO in aqueous and solid phases. Asterisks label parts of the spectra where solvent subtraction artifacts or laser line are present.

reasonable to use the vibration modes of I2·2H2O-cyc model to assign the Raman spectra of TDO in aqueous solution. In the comparison of Raman data for the solid and aqueous phases at 488 nm laser excitation, some differences are found clearly, as shown in Figure 3. In the 500−2000 cm−1 region, compared with the 488 nm Raman spectrum of TDO powder, in aqueous solution, a new peak at 588 cm−1 appears, two Raman peaks (621, 688 cm−1) disappear, and the relative

indicates that the oligomers constructed by TDO and water in cycle are possible structures. Obviously, from density functional theory, UV absorption spectroscopy, and Raman analysis the conclusion can be drawn that TDO powder may form TDO− water oligomers with cyclic structure (I2·2H2O-cyc) in aqueous solution. Figure 4 shows the structures of TDO in solid and aqueous phases. On the basis of the above analysis, it is 3172



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Table 4. Experimental Raman and B3LYP/cc-PVDZ Calculated Vibrational Frequencies for TDO (300−2000 cm−1 Region) Raman shift solid phase

calculated frequencies (scaled by 0.98) I

II

II·H2O

367 410 417 499 525 539 662 850 986 1051 1123 1141 1311 1475 1585 1670

341 427 437 484 580 628 649 694 816 1041 1072 1086 1123 1304 1544 1731

303 365 381 408 424 475 574 587 660 734 750 796 885 995 1090 1119 1302 1381 1550 1635 1736 1718

II·2H2O 305 307 336 356 379 408 421 458 472 555 626 663 672 692 744 805

944 995 1088 1095 1307 1347 1531 1647 1653 1718

I2·2H2O-cyc 313 321 337 338 354 360 426 436 443 449 491 498 585 597 598 604 658 674 747 754 812 817

819 829 925 934 988 992 1053 1055 1135 1136 1157 1159 1417 1422 1519 1524 1622 1623 1662 1662 1694 1715

aqueous phase

488 nm

488 nm

208 nm

descriptions

332 390 502 621 688 831 985 1103 1432 1682

----- (V1) 583 (V2) 808 (V3) 998 (V4) 1121 (V5) 1425 (V6)

348 (V1) 583 (V2) ----- (V3) ----- (V4) 1121 (V5) 1425 (V6)

NH2 twist (V1) NH2−C−NH2 scissor/O−H rock/NH rock (V2) NH2 rock (V3) O−S−O antisym stretch (V4) NH2−C−NH2 symmetric stretch (V5) NH2−C−NH2 symmetric bend (V6)

considered to be the most possible structure of TDO in aqueous solution because its calculated UV absorption spectrum and Raman spectra agree with the experimental spectra very well. Low solubility of TDO in water may be because the hydrogen-bond interaction of TDO powder is too strong to break by water molecule easily and originated from the TDO−water oligomers with cyclic structure. This work will provide some new insight into how molecules of TDO type exist under different conditions.

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Figure 4. Structures of TDO in aqueous and solid phases

ASSOCIATED CONTENT

S Supporting Information *

−1

intensity of four modes (808, 998, 1121, 1425 cm ) greatly changes. Additionally, small shifts are found for V1(NH2 twist), V4(O−S−O antisym stretch),V5(NH2−C−NH2 symmetric stretch), and V6 (NH2−C−NH2 symmetric bend) modes. According to the calculated vibrational frequencies of I2·2H2Ocyc model, 588 cm−1 (V2) in aqueous phase can be assigned as NH2−C−NH2 scissor/O−H rock/N−H rock vibration. Here O−H rock correlates with the water molecule. In aqueous solution, the 208 nm resonance Raman spectrum shows the same characteristic peaks with 488 nm Raman spectrum, only intensity changes. In summary, all research results show that when TDO is solved in water its molecular structure changes significantly, and there are strong interactions between water and TDO molecule.

Full ref 17. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-571- 86843627. Fax: +86-571- 86843627. E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 20707023), Zhejiang Provincial Key Research Project (No. 2010LM106), Zhejiang Provincial Key Innovation Team (No. 2012R10038-02), and Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology.

4. CONCLUSIONS High-level quantum chemistry calculations, UV absorption, and Raman spectroscopy were used to investigate the structure and properties of TDO in aqueous solution effectively. There are strong interactions between water and TDO molecule by interaction energy analysis and comparison of the solid Raman spectrum with the aqueous Raman spectra. In combination with the interaction energy analysis, I2·2H2O-cyc model was

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Structure and Property Investigations of TDO in Aqueous Phase by

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