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Arvin Moser
Rings plus Double Bonds Equivalent (RDBE) (also known as the degree/element of unsaturation or index of hydrogen deficiency) can be determined from a Molecular Formula (MF). The calculation provides insight into the number of rings and/or double bonds and/or triple bonds to expect from an unknown structure. RDBE = (# of C) + 1 –...

Arvin Moser
Depending on the experimental conditions, signals from exchangeable protons such as NH and OH can be present on a 1H NMR spectrum. A key to interpreting a 1H NMR is distinguishing between CHn protons (where n = 1, 2 or 3) and exchangeable protons. Acquiring an 1H-13C 2D NMR experiment can assist in this process,...

Arvin Moser
Without any structural information, we can estimate an upper limit on the number of carbons using the m/z for the molecular ion. For the molecular ion at m/z 386.4 Da (shown in the mass spectrum below), the upper limit on the number of carbons is calculated at 386.4 / 12 = 32.2. Rounding down, we...

Arvin Moser
MS data collected off an instrument is presented as either profile or centroid mode. Shown below are two mass spectra illustrating an ion cluster for profile data and a centroid mass spectrum created from the profile data. In profile mode, a peak is represented by a collection of signals over several scans. The advantage of...

Arvin Moser
On an MS dataset, the x-axis is a measure of the mass to charge (m/z) ratio. Although it is easy to classify a signal as belonging to a singly charged ion (+1 or -1), one needs to be capable of recognizing a multiply charged (+2, -2, +3, etc.) ion as not to misinterpret the MS...

Arvin Moser
I got some great comments from Tony and Gary on the “Dealing with a Mixture…” post that I decided to do a follow-up post. http://acdlabs.typepad.com/elucidation/2008/02/dealing-with-a.html In my lab days, I would routinely workup a newly acquired 1H NMR for an unknown sample prior to acquiring a 2D NMR dataset. One of my goals was to examine...

Arvin Moser
As would-be elucidators dive into an elucidation for an organic unknown, narrowing down a single molecular formula (MF) becomes vital in simplifying the elucidation. However, one hurdle quickly leads into another. As the total atom count adds up, the number of possible isomers increases just as much (See Table and Graph below). I can’t imagine...

Arvin Moser
One of the hardest parts of interpreting an NMR dataset is separating peaks from a mixture of compounds. To illustrate this point, below are the aromatic regions of two 75 MHz 13C NMR run on a mixture of methylbenzene and ethylbenzene in CDCl3. If the mixture exists at an approximate 1:1 ratio (top figure), then...