Structure Elucidation and Spectroscopy Flashcards
(38 cards)
Describe what an inversion point is. Comment on molecules with inversion points.
In a structure with point symmetry, the IP at the center of the molecule such that if two lines are drawn in opp. directions along the same axis, then both line segments intercept identical atoms at the same distance from the IP. Molecules with IPs are NONPOLAR, Dipole vectors cancel each other out.
As symmetry (increases/decreases), the number of signals in the spectroscopic study (increases/decreases).
As symmetry increases, the number of signals in a spectroscopic study decreases.
What information can degrees or units of unstaturation give us?
They give us information about number of rings and/or pi bonds present within a molecule. For every extra pair of bonding e- in a molecule, there is a unit of unsaturation.
Define alipathic.
Alipathic refers to a structure that has no rings or pi bonds. Alkanes are alipathic.
What’s the formula for alipathic alkanes?
CnH2n+2
Equation for degrees of unsaturation?
(2(#C) + (#N) + (#H) - (#X) + 2)/2
Describe IR spectroscopy in detail.
IR is directed at molecule, which absorbs IR, causing transitions b/t vibrational energy levels within it, so that the molecule vibrates more frequently as E is absorbed. This change in vibrating (stretching) b/t atoms causes a change in the dipole moment, which could be monitored. We record the change in the intensity of IR from when it enters the molecule and when it exits.
Describe IR spectroscopy in layman’s terms.
When you bombard a molecule with IR, it causes different bonds to vibrate at distinct frequencies. This is measured in cm^-1.
Relate wave number, wavelength, and energy.
1/λ = Wn α Energy that is absorbed by bond
When is IR most useful?
Functional groups or which functional groups aren’t present.
What are limitations of IR?
Doesn’t tell us where functional group is located or how many of them are in a molecule. Also useful for distinguishing constitutional isomers, but not stereoisomers.
What is the wave number for: O-H; C=O; C=C? CN? CC triple?
OH = 3200 - 3600 cm-1; CO = ~1700; CC = 1650; CN or triple CC = 2100 - 2260.
What occurs with H-bonding and IR spec?
OH groups in IR spec have a broadened peak. Molecules are in continuous random motion in liquid. Groups have strong H-bonds, weak, or none. This results in a random distribution of H-bonds and thus covalent bonds. H-bond weakens the covalent bonds, lowers the E of the bond and thus lowers the E of absorption for the bond.
Which infrared photons affect the vibration energies or a molecule, UV and visible photons affect the ____ energy levels.
Electronic. When UV or visible photons are absorbed by a molecule, an e- is said to be excited from the ground state to an excited state.
When is UV-visible spec most useful?
We typically use only UV-visible spec to analyze molecules with pi-bonds, especially in conjugated systems. UV-visible spec in Ochem focuses on transitions between pi and pi* energy levels. Sigma bonds aren’t analyzed bc it requires A LOT of E. If a UV-visible active compound is involved, it can be used to determine a yield of a reaction.
In UV-visible spec, as the conjugation increases, the wavelength (decreases/increases)?
As the conjugation increases, so does the wavelength of the λmax ( λ of highest absorbance).
In UV-visible spec, how do carbonyls compare with alkenes in terms of absorbance?
Aldehydes and ketones have more intense absorbances that are longer in wavelength than their hydrocarbon counterparts. This is attributed to the n–> pi* transition, possible bc of the lone pairs on the carbonyl O.
Explain NMR.
E in the form of EM radiation is added to the sys. and analyzed in terms of what is absorbed. The E levels that are affected are for the spin of a nucleus in the presence of an external magnetic field. Within a molecule, two identical nuclei may be in diff. electronic environments. So, the difference in their local B fields, caused by the moving e-, manes they won’t require the exact same amount of E to excite the nucleus to a higher E spin state.
Explain NMR in layman’s terms.
If we give H+ some energy (by radio wave absorption), then the H+ can be promoted (flipped) to the higher energy spin, which is opp to the direction of the external B field. This absorption is called resonance. The resonance frequency is the frequency of the radio wave that’s needed to cause a flip in spin. The resonance frequency energy or field strength) of absorption is called the chemical shift.
What is “spin” in terms of NMR?
As the nucleus precesses ( twist due to ext torque), it generates a weak B-field ( just as spinning e- do). When the atomic nucleus has an odd number of protons, the spins cannot pair up to cancel each one another out. For H spec, protons have a spin up or spin down.
Explain broad peaks in NMR.
Protic hydrogens can be distinguished from other signals by its braodness. Broadening results from H-bonding in solution.
What three things must you consider in analyzing NMR Spectrum?
Integral (number of H that make up a signal; splitting pattern ( coupling between hydrogen neighbors); shift value ( determined by local B field caused by lone pairs e- in the motion or electronic density associated with EN atoms);
In a CH2 group of 3-pentanone, how many spin combos does it have? What are they. How does this split a neighboring CH3?
The two H of CH2 have one of four possible spin combinations: up/up, up/down, down/up, down/down. Every CH3 group next to a up/up CH2 has a slightly higher signal. Every CH3 group next to down/down CH2 has a lower. The CH3 signal would appear in a 1:2:1 signal. A Triplet.
The splitting patterns follows the 2n+1 rule. True or false?
It’s the “n+ 1” rule. If there are two neighboring non-equivalent H, then that H gets split into three. It’s a triplet. 2 +1 = 3.
