University of Wisconsin - Green Bay

Research Council


Franklin Chen

Franklin Chen

Spring 2013

"Electronic Origin of Acidity of Oxyacids"

Grant for Integrating Research and Teaching

Final Report: "This proposal proposes to conduct computation research to study the acidity. Acidity and basicity are well suited for computation research. Several published works in Journal of Chemical Education1 have discussed specific class of chemistries in the framework of ab initio quantum chemistry which have offered pedagogical values to the undergraduate students. This author2 has also demonstrated the correlation between pKa (a numerical value for ranking the acid strength) and calculated ΔG values (free energy change between base and acid) in 18 oxy-acids. The calculation was conducted using Gaussian, Gauss-view, and NBO programs which are relatively expensive and not readily accessible to undergraduate students. Those programs allow me to frame the conceptual questions of acid-base character in terms of a well-defined computational protocol that can be independently pursued by students and researchers alike, making use of modern Natural Bond Orbital (NBO) methods to analyze the computation results. The proposal promises to set-up computation infrastructure at UW-Green Bay campus. This infrastructure includes: GAMESS from the Iowa State University, PC-GAMESS from the Moscow University , Web-MO from Michigan and finally NBO Pro program. With the help of the grant money, he has purchased the license key and NBO 6.0 program with a cost of approximately $800 to complete the infrastructure of GB capability of conducting modern computation research for undergraduate students starting even at a sophomore level. The proposal also promises to recruit students to work on acidity of moderate to strong oxy-acids such HNO2, HNO3, HClO, HClO2, and HClO3. Since I did not succeed in recruiting students to conduct computation work, I made a detour for this promise. I work primarily on my own, however, on the acidity of saturated alcohols, XHnOH as well as CFH2(CH2)nOH focusing on the effects of electronegativity and size of X and the effects of the separation of F from the OH group. Both classical induction effects (which increase acidity as electronegativity ΞX adjacent to O increases), and quantum mechanical hyperconugation (vicinal nO→σXY* donor-acceptor interaction, which increases acidity as neighboring ΞX decreases) are found to affect acidity, but the latter is clearly dominant in most of the acids. For saturated alcohols from the 3rd period (i.e. SiH3OH, PH2OH, and HSOH) reduction of the steric strain in the base form also contributes to the acidity. Some of the results are shown as follows: Figure 1 shows Dependence of DEL on natural electronegativity when X is in the 2nd-period (i.e. for CH3OH, NH2OH, HOOH) and when X is in the 3rd-period (i.e. for SiH3OH, PH2OH, SHOH, and ClOH series. A quite interesting finding is that when X is in the third period, the saturated alcohols are more acidic than those alcohols when X is in the second period despite the electronegativity ΞX for X are lower than those corresponding values when X is in the second period. This implies that SiH3OH is more acidic than CH3OH, PH2OH is more acidic than NH2OH and SHOH is more acidic than HOOH, despite that Si, P, and S have lower electronegativity than C, N and O. [ Electronegativity for Si, P and S are 1.78, 2.06, and 2.42; for C, N and O are: 2.60, 3.07, 3.48 respectively.] This acidity is due to primarily more flexibility of the X-O-H bonds when X is in the 3rd period as compared to the Y-O-H bonds when Y is in the 3rd period. This results lower steric hindrance (Table 1) and higher hyper-conjugative interactions of nOàσ*XH (Figure 3). I also made investigations looking into the effects of substituting a strong electronegative element, F for H on the acidity of the saturated alcohols. This includes documenting the NBO descriptors of CH2FOH, CHF2OH, CF3OH and CH3OH as well as studying the effects of F-substitution on acidity as the distance between F and O is increased from CH2FOH to CH2F CH2OH and from CH2F CH2OH to CH2F CH2CH2OH. To normalize the effect of the alkyl chain on acidity, we also include C2H5OH and C3H7OH in the study. As the number of F substitutions increases from CH3OH to CF3OH, so is the increase of acidity in complete linear manner as ΔEvert(W) decreases from 0.5086 a.u in CH3OH to 0.4503 a.u. in CF3OH with a R2=0.9885 (Fig.4). Adding additional CH2 group to CFH2OH decreases the acidity. The differences of ΔEvert(W) values (ΔΔEvert(W)) between the ‘parent’ alkyl alcohols and their F-substituted alcohols at the distal alkyl group diminishes as additional CH2 groups are added to CFH2OH. Figure 6 shows the acidity differences between CFH2(CH2)nOH and CH3(CH2)nOH. ΔΔEvert(W) , ΔΔEL(W) and ΔΔENL(W) are defined as differences of the ΔEvert(W), ΔEL(W), and the ΔENL(W) values between the fluorine-substituted alkyl alcohols and the straight alkyl alcohols. Even this work has made detour from our original promise, we made important discoveries that includes: (1) SiH3OH is more acidic than CH3OH, PH2OH is more acidic than NH2OH, and HSOH is more acidic than HOOH despite the electronegativity for Si, P, and S are smaller than those for C, N and O. The acidity for SiH3OH series is due to less steric strain comparing to CH3OH series. (2) While both the inductive and hyper-conjugative effects contribute to the acidity of saturated alcohols that include F substitution, both influences diminishes as the distance between F and O increases. This author plans to made a presentation to UW-Green Bay audience for this work in the fall of 2014. Hopefully, after that presentation, I can successfully recruit students to conduct work as what I had promised in the proposal."

Spring 2008

"Sensor Development Based on Molecularly-Imprinted Polymer"

Grant in Aid of Research

Fall 2006

"Develop methods and chemicals to enhance Contrast of Fingerprints and Preserve them"

Grant in Aid of Research

Spring 2003

"Purchase SigmaPlot 8 Data Analysis Software"

Grant in Aid of Research