Cumulative exams Analyt Biochem Org Inorg Phys 2018

Department of Chemistry Cumulative Examinations December 8, 2018 You тау choose to answer any exam from any area covered in the examination booklet. Each exam may contain multiple parts. You тау answer тоге than one exam but each exam is scored separately and is treated as an individual examination result. Thus, answering parts of two exams with а score of 50% would not yield а 100% grade for this cumulative exam. lnstead you would receive 50% оп each examination attempted. This booklet contains five examinations. 1) Analytical Cumulative Examination, Page 1 2) Biochemistry Cumulative Examination, Pages 2-3 Page 4 lnorganic Cumulative Examination, 3) 4) Organic Cumulative Examination, Pages 5-6 5) Physical Cumulative Examination, Pages 7-8 SEPARATE BOOKLET lf you take the PChem exam, you must submit both the Ыuе book and the question packet with your Purdue 1D written оп both documents. Оп your examination booklet: 1) 2) 3) 4) Print your student I D number. Print the Exam Booklet number. Print the question number you are answering. Print the Exam Date. Do not write your пате anywhere оп the examination booklet. Each exam will Ье scored anonymously. lf you attempt тоге than one exam, уои must use а separate examination booklet for each examination. When you complete the examination, return the examination and your answer booklet to the proctor. Exam results will Ье posted оп bulletin board #2В оп the north side of the hall near BRWN 2124. RESEARCH 1i1lйФil CHEMICAL PHYSICS Femtosecond x-ray spectroscopy of ап electrocyclic ring-opening reaction Aшlrew R. Attar,1'2 Aditi Bhattacheijee,1'2 С. D. Pemmaraju,3,4 KiJ:sten Schпorr,1•2 Кristina D. Closser,3 David Prende1·gast,3 Stephen R. Leone1•2•5* The ultrafast light-activated electrocyclic ring-opening reaction of 1,3-cyclohexadiene is а fundamental prototype of photochemical pericyclic reactions. Generally, these reactions are thought to proceed through ап intermediate excited-state minimum (the so-called pericyclic minimum), which leads to isomerization via nonadiabatic relaxation to the ground state of the photoproduct. Here, we used femtosecond (fs) soft x-ray spectroscopy near the carbon K-edge (-284 electron volts) оп а taЬle-top apparatus to directly reveal the valence electronic structure of this transient intermediate state. The core-to­ valence spectroscopic signature of the pericyclic minimum observed in the experiment was characterized, in comblnation with time-dependent density functional theory calculations, to reveal overlap and mixing of the frontier valence orbltal energy levels. We show that this transient valence electronic structure arises within 60 ± 20 fs after ultraviolet photoexcitation and decays with а time constant of 110 ± 60 fs. т he light-activated electrocyclic 1ing-opening of the 1,3-cyclohexadiene (CHD) chromo­ phшe is responsiЬle for а c111cial step in the photoЬiological synthesis of vitamin D3 in the ski11 (1-3) and underlies nurnerous optoelectronic. applications jn optical switching (4), photochromic devices (5), and nanomechanical motors (6). Тhе photoinduced isomerization of this particular chromophore has also played а critical role in the development and coпobora­ tion of the Woodward-Hoffman ru1es that govel'n the stereochemical fate of pericyclic reactions (7, 8) more genei-ally. Тhе reaction scheme of photo­ chemical pericyclic reactions is postulated to involve nonadiabatic transit through an intermediate ex­ cited state of the same symmetry as the electronic grotmd state, implicating а complex intel'play be­ tween nuclear and electronic degl'ees offreedom (9). Serving as а model of this uЬiquitous process, the photoinduced ring-opening of СНD has been the subject of extensive expe1imental and theoretical investigations (10-14), which have provided the currently developed mechanism presented sche­ matically in Fig. lA. Following ultraviolet (UV) photoexcitation to а lВ excited state via а strong symmetry-allowed 1t---->1t* transition, the photochemical ring-opening reaction is thought to proceed to а dark 2А ex­ cited state (11) near а surface crossing, or con­ ical intersection, between the lВ and 2А states 1Department ofChemistry, University of California, Berkeley, СА 94720, USA. 2Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, СА 94720, USA. 3The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, СА 94720, USA. 4Theory lnstitute for Materials апd Епегgу Spectroscopies, SLAC National Accelerator Laboratory, Мепlо Рагk, СА 94025, USA. 5Department of Physics, Uпiversity of California, Berkeley, СА 94720, USA. *Corresponding author. Email: srl@berkeley.edu Atta1· et al., Science 356, 54-59 (2017) 7 Ap1·i\ 2017 0abeled СП) after ~55 fs (12, 13) and, subsequently, through the so-called pericyclic minimurn in this intermediate state (11). From an electronic sb:uc­ tщe perspective, the 2А state сап Ье described Ьу а douЬly excited electronic configuration, as shown in the inteпnediate.orЬital diagram i11 Fig. lВ (9, 14). According to recent simulations (15), the strict separation into а bright lВ state and а dark 2А state may not Ье exact; however, this labeling convention conveniently diffel'en­ tiates the characteristically distinct regions of the excited-state potential energy surface. As the wave packet moves along the reaction coordinate with concur!'ent conversion of the photon energy into nuclear dynamics, а !'eduction i11 the 1t-1t* splitting is expected nea.i· the pericyclic minimurn, consistent with the narrowing gap between the gi·ou11d­ and excited-state potential energy surfaces. !11 this region, а conical intei-section between tl1e 2А excited-state and lA groнnd-state potential enei-gy surfaces (CI2) pi-ovides the critical outlet for the wave packet to undei-go nonadiabatic re­ laxation onto thelA tш1sitio11 state region (11, 14). From here, the wave packet Ьifurcates toward eithe1· the 1,3,5-hexatriene (НТ) photoproduct 01· back toward the ring-closed CHD (13, 15). Тhе aпival time at the initial ring-opened НТ photo­ product has been previously l'eported to Ье ~140 fs (12, 14). А key to нnderstanding this photochem­ ical mechanism, which involves i-apid exchange between electronic and nнclear motion, is to di­ rectly characte1ize both the time-evolving molec­ ular st111ctнre and the coнpled evolution of the valence electronic stп1cture, especially through the crucial inte1111ediate excited state. Although time-i-esolved spectroscopic studies have allowed tempol'al traciпg of the excited­ state dynamics (10, 12-14) and receпt femto­ secoпd х-!'ау scatte1ing experiшents have even revealed the time-evolving molecular st1·uctщe directly (16), а direct chai:acterization of the tl'an­ sieпt va1ence electronic stшctшe in the intermediate excited-state regioп has remained elusive. Не!'е, we l'eport femtosecond soft x-ray absorptioп spec­ t1·oscopy near the carbon K-edge with spect1·0scopic analysis froш time-dependent density functional theory (TDDFГ) calculations of the x-l'ay spectra to directly reveal the evolution of the valence electroпic structшe dщing the photo­ chemical ring-opeпing reaction in !'eal time. Non­ adiabatic nюlecuJar dyna.inics (NAМD) siшulations of the entire reactioп path wel'e performed and the time-resolved х-гау absorption spectrum (ГRXAS) calculated at each time point via 'IDDFГ (17). Iп gene!'al, х-шу absoi-ption spectroscopy is а powerful tool fol' exploring the electronic stшc­ ture of molecules with ele111e11t specificity (18) and, when applied with ult:J:afast purnp-proЬe tech­ пiques, сап provide detailed iпfo1matioп rega.i·d­ ing the time evolнtion of oxidatioп states, spin states, and chemical enviroпments of specific atom­ ic sites during photoindнced processes (19-29). Femtosecoпd х-гау (>100 eV) spectюscopic studies have Ьееп mostly rest:Jicted to large-fucility syn­ chrotrons orfree-electron laser soшces (20, 24, 25) and only i-ecently extended to taЬ!e-top systeшs (30). Here, we нsed а taЬ!e-top, high-harmonic x-ray sощсе to pl'obe the UV photoinduced ring­ opening reaction of СНD via soft x-ray femtosec­ ond tl'ansient absorption spectюscopy near the саrЬоп K-edge (~284 eV), thнs demonstrating femtosecond optical рнmр, x-ray р1·оЬе spectros­ copy on а taЫe-top apparatus. B1ietly, а 266-пm ' purnp puJse (4 µJ) was нsed to photoexcite the CHD molecнles to the lВ state Ьу pl'omotion of ап electronfrom the 21t blghest-occupied (НОМО) to the lJt* lowest-unoccнpied (LUМO) molecula1· orЬital, initiating the electrocyclic l'ing-opeпing i-eaction. Broadband soft x-ray pнlses (~160 to 310 eV, sнЬ-50 fs) prodнced via high-harmonic geпeration (ННG) were нsed to acquire х-гау aЬ­ sorptioп snapshots of the 1·eactiпg mo�ecнle in real time (17) (figs. Sl and S2). An independent in situ cross-co1тelation between the UV and soft х-шу pulses was used to find time zero and the in­ stп1111eпt response functioп (IRF) of the experi­ ment, giving а full-width at half-maximurn ofl20 ± 15 1s (figs. S3 to S5). In Fig. 2А, the static near-edge x-ray absorp­ tion fine structure (NEXAFS) spectшm of CHD is plotted a.lld compared to the co1тesponding1DDFГ calculation. Here, we focus on the pl'e-edge region of the spectГLm1 (see fig. S6 for the full static NEXAFS spectrurn пр to 296 eV). Тhе pre-edge i-esonaпces, labeled peaks Х and У, each consist of several ove1·lapping transitions into unoccupied molecнlar va!ence orЬitals. Peak Х comp1ises саr­ Ьоп ls---->l1t* LUМO t!'ansitions localized on the sp2-hybridized carbon sites; peak У comprises ls---->cr*(C-C), ls---->21t*, and ls-->cr*(C-H) transi­ tions (31). UV photoexcitation promotes an elec­ tron from the НОМО (21t) of CHD to the LUМO (l1t*), immediately nюdifying the electronic stшc­ ture апd, therefore, the NEXAFS spectrum. As the potential energy imparted Ьу the photon is coпverted into kinetic energy of the nнclei, tl1e 1 of 5