CHEM 1810 - Lecture 19

Lecture 19 Spectroscopy Hydrogen absorption spectrum: Shining a broad range of light frequencies through a sample (H), and specific ones missing/absorbed Transmittance is the same but inverse Higher frequency: ↑ accuracy, ↓ λ (↑ momentum/photon) X-rays: Enough E to knock the e completely out of molecule Ionization, diffraction (XPS): Info about orbital E Gamma and nuclear changes: E changes due to kinetic structure UV and visible light: Info about electronic structure IR (vibration, micro): E through rotation (electron spin, EPR) Microwaves: E through rotation (electron spin, EPR) Radiowaves: Nuclear spin (NMR) in magnetic field Kinetic Energy Storage Translation: Motion of whole molecules with center of mass traveling through space Adding E to a system by raising temperature involves ability of molecules to store E Translation can happen along the x, y, and z axes independently ↑ temp, ↑ speed (displacement/time) Rotation: Motion of a molecule as a whole around an axis passing through the center of mass (COM does not change/move) 3 degrees of freedom Can be broken down independently A molecule can move in a composite of translation & rotation For every additional degree of freedom, it's an additional way to store kinetic energy (↑ DOF, ↑ ability to store KE) A rotation about the z-axis for a linear molecule (bonding axis) is undetectable; not a way to store E; be no change in inertia Linear molecules have 4 ability to store E than 3D ones 3) Vibrations: motion of parts of a molecule (ex atoms) relative to other parts within a molecule, about a COM bond lengths oscillate more DOF; more atoms in a molecule, more ways to vibrate & store E in the gas phase: total DOF = 3N (N= # of atoms) vibrational modes = 3N-6 (3N-5 if linear) ex. water: 3 translational & 3 rotational DOF liquid: bond angle inc. and dec. get longer shorter Symmetric Stretch Asymmetric Stretch bend V1 V2 V3 orgas Solid: x less movement vibrations molecule moves as a whole; not individual bonds multiple vibrations may occur at once -> more complicated vibrations or bending the greater the number of DOF, the more ways to store energy; the greater the heat capacity heat capacity (C_v, C_p): how much heat is required to raise the temp of a substance by one degree adding the same amount of heat to a larger material (↑ heat capacity) changes its temp less than a smaller one; determined, in part, by how it's energy is being stored; spreads E out in more ways thermal energy is coupled/equilibriated; adding energy into one form of storage distributes to the others most DOF same resultant temp; same # of atoms; same # of DOF lower temp than Ar/He; more ways to store E (can rotate and vibrate in addition to translate) & ↑ atoms. infrared spectroscopy: everything around CoM is moving "complex vibrational motions" can be separated into "normal modes" of vibration. each normal mode has its own, unique frequency of vibration determined by bond masses and bond strengths infrared spectroscopy allows us to determine these frequencies & molecular structure info. note: in sym. stretch, O2 slightly moves up to maintain CoM (may be dominated by motion around a couple atoms (closely related to that bond) diatomic molecules treat like a harmonic oscillator ν = frequency κ = force constant μ = reduced mass μ = (M1 * M2)/(M1 + M2) (bigger mass -> ↓ν) directly related to bond strength more E to break, stiffer & more "glue" harder to break / displace