• Bulletin of the Korean Chemical Society
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• 제35권8호
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• pp.2371-2374
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• 2014

A kinetic study is reported on SNAr reaction of 1-fluoro-2,4-dinitrobenzene with a series of primary amines including hydrazine in $H_2O$ at $25.0^{\circ}C$. The plots of $k_{obsd}$ vs. [amine] are linear and pass through the origin, indicating that general-base catalysis by a second amine molecule is absent. The Br${\o}$nsted-type plot exhibits an excellent linear correlation with ${\beta}_{nuc}$ = 0.46 when hydrazine is excluded from the correlation. The reaction has been suggested to proceed through a stepwise mechanism, in which expulsion of the leaving group occurs after the rate-determining step (RDS). Hydrazine is ca. 10 times more reactive than similarly basic glycylglycine (i.e., the ${\alpha}$-effect). A five-membered cyclic intermediate has been suggested for the reaction with hydrazine, in which intramolecular H-bonding interactions would facilitate expulsion of the leaving group. However, the enhanced leaving-group ability is not responsible for the ${\alpha}$-effect shown by hydrazine because expulsion of the leaving group occurs after RDS. Destabilization of the ground-state of hydrazine through the electronic repulsion between the nonbonding electron pairs is responsible for the ${\alpha}$-effect found in the current $S_NAr$ reaction.

• Bulletin of the Korean Chemical Society
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• 제33권9호
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• pp.2971-2975
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• 2012

Second-order rate constants $k_N$ have been measured spectrophotometrically for nucleophilic substitution reactions of t-butyl 4-pyridyl carbonate 8 with a series of alicyclic secondary amines in $H_2O$ at $25.0{\pm}0.1^{\circ}C$. The Br${\emptyset}$nsted-type plot for the reactions of 8 is linear with ${\beta}_{nuc}$ = 0.84. The ${\beta}_{nuc}$ value obtained for the reactions of 8 is much larger than that reported for the corresponding reactions of t-butyl 2-pyridyl carbonate 6 (i.e., ${\beta}_{nuc}$ = 0.44), which was proposed to proceed through a forced concerted mechanism. Thus, the aminolysis of 8 has been concluded to proceed through a stepwise mechanism with a zwitterionic tetrahedral intermediate $T^{\pm}$, in which expulsion of the leaving-group from $T^{\pm}$ occurs at the rate-determining step (RDS). In contrast, aminolysis of benzyl 4-pyridyl carbonate 7 has been reported to proceed through two intermediates, $T^{\pm}$ and its deprotonated form $T^-$ on the basis of the fact that the plots of pseudo-first-order rate constant $k_{obsd}$ vs. amine concentration curve upward. The current study has demonstrated convincingly that the nature of the leaving and nonleaving groups governs the reaction mechanism. The contrasting reaction mechanisms have been rationalized in terms of an intramolecular H-bonding interaction, steric acceleration, and steric inhibition.

• Bulletin of the Korean Chemical Society
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• 제34권8호
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• pp.2251-2255
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• 2013

Second-order rate constants ($k_{HOO^-}$) for the nucleophilic substitution reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with $HOO^-$ in $H_2O$ have been measured spectrophotometrically. The ${\alpha}$-nucleophile $HOO^-$ is 10-70 times more reactive than the reference nucleophile $OH^-$ although the former is ca. $4pK_a$ units less basic than the latter, indicating the ${\alpha}$-effect is operative. The Bronsted-type plot for the reactions of 4a-4i with $HOO^-$ is linear with ${\beta}_{lg}=-0.51$, a typical ${\beta}_{lg}$ value for reactions which were reported to proceed through a concerted mechanism. The Yukawa-Tsuno plot is also linear with ${\rho}=1.40$ and r = 0.47, indicating that a negative charge develops partially on the O atom of the leaving group, which can be delocalized to the substituent Y through resonance interactions. Thus, the reactions have been proposed to proceed through a concerted mechanism. The magnitude of the ${\alpha}$-effect (i.e., the $k_{HOO^-}/k_{HO^-}$ ratio) decreases linearly as the leaving-group basicity increases. It has been concluded that solvation effect is not solely responsible for the ${\alpha}$-effect found in this study but the transition-state stabilization through an intramolecular H-bonding interaction is also responsible for the ${\alpha}$-effect.

• Bulletin of the Korean Chemical Society
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• 제14권2호
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• pp.191-195
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• 1993

Molecular orbital calculations at the extended Huckel level have been carried out for an electron deficient cluster, $Tl_3(FeL_3)_2{(FeL_4)_3}^{-3}$, where L=CO or $H^-$. The LUMO, $2a_2$", is destabilized by the secondary interaction of the LUMO with $1a_2$" on $(FeL_3)_2$ fragment. This is one of six skeletal bonding orbitals which are associated with $Tl-FeL_3$ bonds. Overlap population analysis has been applied to account for two kinds of Tl-Fe bonds. Replacement of the terminal $C_{3v}$, $FeL_4$, by the $C_{2v}$, $FeL_4$ units in cluster results in slight energy stabilization of the cluster.

• 대한화학회지
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• 제59권2호
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• pp.117-124
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• 2015

Hydrogen polyoxide, $H_2O_n-H_2O_m$(n=1-4, m=1-4) 이중합체(dimer)의 분자 구조 변화에 따른 수소결합(H-bonding) 효과를 순 이론적(ab initio) 방법과 밀도 범함수 이론(DFT)으로 계산하였다. 분자 구조는 B3LYP, CAM-B3LYP, MP2의 양자역학적 방법들을 사용하여 최적화하였으며, 진동주파수를 계산하여 최저에너지(true local minimum) 구조인 것을 확인하였다. 보다 정확한 수소결합 에너지(${\Delta}E$) 계산을 위하여 CCSD(T) 이론수준에서 한 점(single-point) 에너지 계산을 하였으며, 영점 진동에너지(ZPVE) 보정과 바탕집합 중첩에러(BSSE) 보정을 하였다. CCSD(T)/cc-pVTZ 이론 수준에서 $H_2O_4-H_2O_3$이 8.18 kcal/mol로 가장 강한 결합을 나타내었으며, 물 이중합체($H_2O-H_2O$)는 3.00 kcal/mol로 가장 약한 결합에너지를 나타내었다.

• Macromolecular Research
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• 제17권5호
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• pp.307-312
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• 2009

Core-shell hydrogel nanocomposite was fabricated by encapsulating a silica-gold nanoshell (SGNS) with poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AAc) copolymer. The oleylamine-functionalized SONS was used as a nanotemplate for the shell-layer growth of hydrogel copolymer. APS (ammonium persulfate) was used as a polymerization initiator to produce a hydrogel-encapsulated SGNS (H-SGNS). The amounts of NIPAM (N-isopropylacrylamide) monomers were optimized to reproduce the hydrogel-encapsulated SGNS. The shell-layer thickness was increased with the increase of polymerization time and no further increase in the shell-layer thickness was clearly observed over 16 h. H-SGNS exhibited the systematic changes of particle size corresponding to the variation of pH and temperature, which was originated from hydrogen-bonding interaction between PNIPAM amide groups and water, as well as electrostatic forces attributed by the ionization of carboxylic groups in acrylic acid.

• Bulletin of the Korean Chemical Society
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• 제9권3호
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• pp.121-126
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• 1988

The molecular interaction of $Mo_2(OR)_4({\mu}-OR)_2$$ unit has been studied for carbonyl and acetylene ligands by means of extended Huckel calculations. We have extended the extended Hckel calculations to unknown apex bridged $Mo_2(OR)_6({\mu}-X)$ complexes (X = $SO_2$H, and Cl) in order to compare the stability of the complexes. In the $C_2H_2$and H apex bridged complexes, one finds a relatively small HOMO-LUMO gap. However, both complexes are stabilized due to Jahn-Teller distortion. The stability of both complexes is comparable to that of $Mo_2(OR)_6(CO)$ complex. The comparisons and predictions in the stability of known and unknown complexes are the subject of this study.

• Bulletin of the Korean Chemical Society
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• 제17권9호
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• pp.845-849
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• 1996

Bis(N,N-dimethyl-2-aminomethylthiophenium)tetrahalocuprate(Ⅱ) salt, (dmamtH)2CuCl4 and (dmamtH)2CuBr4 were prepared and characterized by spectroscopic (IR, UV-Vis, EPR, XPS), electrochemical method, and magnetic susceptibility measurement. The experimental results reveal that the compounds have pseudotetrahedral symmetry around copper(Ⅱ) site due to the steric hinderance of the bulky 2-(dimethylaminomethyl)thiophene in the complex. The N-H…Cl type hydrogen bonding is expected in (dmamtH)2CuCl4 from the XPS and IR data. Magnetic susceptibility data show that both of the compounds follow Curie-Weiss law in the range of 77-300 K with negative Weiss constant exhibiting antiferromagnetic interaction between copper(Ⅱ) ions in solid state.

• 한국자기공명학회논문지
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• 제26권4호
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• pp.46-50
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• 2022

Antifreeze proteins (AFPs) bind to the ice crystals and then are able to inhibit the freezing of body fluid at subzero temperatures. Type III AFPs are categorized into three subgroups, QAE1, QAE2, and SP isoforms, based on differences in their isoelectric points. We prepared the QAE2 (AFP11) and SP (AFP6) isoforms of the notched-fin eelpout AFP and their mutant constructs and determined their temperature gradients of amide proton chemical shifts (𝚫δ/𝚫T) using NMR. The nfeAFP11 (QAE2) has the distinct 𝚫δ/𝚫T pattern of the first 3₁₀ helix compared to the QAE1 isoforms. The nfeAFP6 (SP) has the deviated 𝚫δ/𝚫T values of many residues, indicating its backbone conformational distortion. The study suggests the distortion in the H-bonding interactions and backbone conformation that is important for TH activities.

• 대한화학회지
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• 제38권3호
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• pp.241-245
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• 1994

산화성 첨가반응을 이용하여 실라트란(분자내 Si-N고리횡단 상호작용을 갖는 5-배위의 규소화합물)의 규소원자와 주족원소간에 Si-M 결합작용이 있는 새로운 이종다핵 화합물을 합성하려고 시도하였다. 1-브로모실라트란(1a)과 $SnBr_2를 반응시켜 노란색(2a)과 흰색(2b)의 고체 혼합물을 얻었고 용해도 차에 의해 각각을 분리하여 ^1H-NMR, ^{29}Si-NMR, ^{119}Sn-NMR, Mass스펙트럼으로 규명한 결과 노란색 화합물만이 Si-Sn결합을 갖는 것으로 판명되었다. SnBr_2를 1-브로모-3,7,10-트리메틸실라트란(1b)과도 반응시켜 Si-Sn 결합을 갖는 화합물, N[CH_2CH(CH_3)O]_3SiSnBr_3(CH_3OH)_2(3)$을 분리하여 여러가지 방법으로 확인하였다.