ochem 2 Flashcards
A bond with no ___ will not be detected by IR
dipole
The vibration frequency is determined by :
strength of the bond
molecular weight of the bonded atoms
Describe the similarities between vibrational frequency in IR spectroscopy and a mass- spring system. How do bond strength and molecular weight impact frequency?
We can compare a vibrating bond (in only one dimension, ignoring swaying motions) to a mass on a spring. The bond roughly follows Hooke’s Law and either atom can be compared to the mass. A stronger bond would be analogous to a spring with a larger spring constant (k), and a weaker bond would be analogous to a spring with a smaller spring constant. The molecular weight of the atom is analogous to the size of the mass on the spring. Following the principles dictated by Hooke’s Law, a stronger spring (larger k) increases oscillation frequency according to: f = (1/2pi) √(k/m). A weaker bond would therefore vibrate at a lower frequency. A smaller atom is associated with a higher vibrational frequency and a larger atom is associated with a lower vibrational frequency.
What is the IR absorbance for a carbonyl (C=O)
1700 cm-1 (sharp,deep)
What is the IR absorbance for an alcohol?
3300 cm-1 (broad, separate from CH)
What is the IR absorbance of a saturated alkane, CH?
2800 cm-1 sharp, deep
What is the IR absorbance of carboxylic acid?
3000 cm-1 (broad, overlaps CH)
What is the IR absorbance of an amine?
3300 cm-1 , (broad shallow)
What is the IR absorbance of an amide?
3300 cm-1 (broad, deep)
What is the IR absorbance of a nitrile?
2250 cm-1 (sharp, deep)
Molecules containing only ___ bonds show low or no UV absorbance in UV spectroscopy
single
What type of bonds absorb UV strongly?
double and triple bonds
___ absorb UV even more strongly than do isolated double or triple bonds
conjugated systems
The greater the degree of conjugation, the farther ___ the species will absorb
right (higher wavelength)
The height of each peak gives the relative ___ of that fragment in mass spectrometry
abundance
What is the parent peak of a mass spectrometry?
represents the original molecule minus one electron
What is the base peak of a mass spectrometry?
is the most common fragment and is usually the most stable fragment generated. It is defined as existing at 100% relative abundance. In other words, it will be the highest peak on the spectra and the height of all other peaks will be a function of how abundant that fragment is compared to the base peak.
an atom must have either a __ atomic number or a __ mass number to register on an NMR (or MRI)
odd, odd
What do the peaks in H-NMR spectras mean?
represent all of the hydrogens in a molecule that share an indistinguishable chemical environment
What does the area under the peak of an H-NMR represent?
representation of the number of hydrogens accounted for by that peak
absorbance range in a H-NMR spectra is__
0-12 ppm
12 ppm in a H-NMR is __
downfield= deshielded
0 ppm is __ in a H-NMR spectra
upfield = shielded
What is spin splitting in HNMR spectras?
the presence of “neighbors” (non-equivalent hydrogens attached to a neighboring carbon) causes splitting of the peak. The peak for a set of equivalent hydrogens will be split into exactly n + 1 sub-peaks