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CHEM test 3 text notes-3

CHEM test 3 text notes-3 - CHEM TEST 3 Text Notes 14.7...

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CHEM TEST 3 Text Notes 14.7 Molecular Spectroscopy: An Introduction spectroscopy – the study of the interaction of electromagnetic radiation w matter electronic transition is when molecules absorbs or emits a photon and go from a lower energy to a higher electronic energy state, or vice versa. It can be described approximately as a change from one electron configuration to another. *Typically, electronic transitions require photons in the UV or visible regions of the spectrum. Molecules can also undergo vibrational and rotational energy transitions Molecules rotate and have quantized rotational energy levels whos spacings correspond to the energies of photons in the microwave region. A molecule can undergo - a pure rotational transition (change rotational levels but remain in the same vibrational and electronic states) - a vibrational transition that may also involve a simultaneous rotational transition - an electronic transition that may involve a simultaneous vibrational and/or rotational change 14.8 Electronic Spectroscopy The electronic spectrum of a molecule – typically occurs in the UV or visible region of the electromagnetic spectrum. Plots the quantity of radiation absorbed versus the wavelength of the radiation, showing peaks (maxima) at wavelengths where the photons have an energy that matches an energy gap in the molecule. Electronic transitions from bonding to antibonding molecular orbitals are often encountered. PE curve for ground state will be quite different from that of excited state bc less bonding electron density is found in the exited state. *The majority of electronic transitions in molecules occurs in the UV region of the spectrum bc energy separations of electron states typically correspond to the energies of photons in the UV region. Some molecules have electronic energy separations that correspond to radiation in the visible regions - coordination compounds that contain transition metal ions - molecules w long chains of carbon molecules that have alternating double bonds conjugated molecules – substances w alternating double bonds
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As the conjugated system gets longer, the electron energies get closer together, and the light absorbed corresponds to longer wavelengths Color of a molecule is given by the “sum” of the colors that remain unabsorbed Photochemistry – study of the chemistry of electronically excited molecules (excited state usually more chemically reactive than ground state) Lasers are very important for electronic spectroscopy because they can deliver large quantities of energy at very precise wavelengths and the power can be delivered in very short bursts (10 -15 seconds in duration) 14.9 Vibrational Spectroscopy Figure 14.58 For small displacements from Re (equilibrium bond distance), the bond can be treated as a harmonic oscillator F= -k (R-Re) F= force k = force constant R= actual bond length When the stretched bond is released (if the system behaves like a classical harmonic
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